Return to Answer

(Note: please see my summary at the end of my answer, I don't have time at the moment to rewrite to fix apparent lack of understanding of what I'm trying to convey.
Instead of downvoting I'd appreciate a comment suggesting improvements!)

This question is quite similar to not remembering a persons name after interacting with them for too long to still be asking for their name.

• Ask outright about their biological sex (!)
  (preferrably within a conversation easing into the topic)
  I would do this, but in your case I recommend the second option, unless you really,really need to know, in which case it seems that it does matter to you after all
  (this one might be awkward and / or surprise / offend your partner)

• my recommendation: wait until a makeout session leads to more intimacy to find out
  (since you're apparently comfortable making out with the person and seem a bit shy to approach the topic, why not simply wait)

• wait until the person reveals their sex on their own accord - basically a variation of my recommendation (whichever comes first will reveal the sex)

Non binary pronouns (them,they,ze etc.) for example are not even a noteworthy topic (and are quite a bit more problematic to implement) especially in non English speaking countries here, since many languages have male and female ingrained into them in prefixes or suffixes for example in addition to the pronouns to address the person.
Some transgender people chose the gender neutral pronoun "it" surprisingly, despite being a bit strange to refer to a person as "it".

EDIT:

In light of the downvotes and Arwen Undómiel's comment I'd like to summarize my reasons for my recommendation (as I thought I already laid out above):

• Asking flat out about gender identity/pronoun as was suggested is not as common in Europe as the USA.
  This is also why I said I'd ask for their biological sex, since OP asked to know about man vs woman.
• The person is not outspoken LGBT activist, so they might not even consider there to be ambiguity regarding their sex,causing surprise/embarrassment.
• OP seems a bit shy in approaching the topic.
• OP is comfortable to make out with the partner without knowing their sex/gender identity.
• OP stated,the result wouldn't really matter either way.
• OP seems to be bisexual,ergo either sex would be fine.

This question is quite similar to not remembering a persons name after interacting with them for too long to still be asking for their name.

• Ask outright about their biological sex (!)
  (preferably within a conversation easing into the topic)
  I would do this, but in your case I recommend the second option, unless you really, really need to know, in which case it seems that it does matter to you after all
  (this one might be awkward and / or surprise / offend your partner)

• my recommendation: wait until a makeout session leads to more intimacy to find out
  (since you're apparently comfortable making out with the person and seem a bit shy to approach the topic, why not simply wait)

• wait until the person reveals their sex on their own accord - basically a variation of my recommendation (whichever comes first will reveal the sex)

Non binary pronouns (them, they, ze etc.) for example are not even a noteworthy topic (and are quite a bit more problematic to implement) especially in non English speaking countries here, since many languages have male and female ingrained into them in prefixes or suffixes for example in addition to the pronouns to address the person.
Some transgender people chose the gender neutral pronoun "it" surprisingly, despite being a bit strange to refer to a person as "it".

To summarize my reasons for my recommendation:

• Asking flat out about gender identity/pronoun as was suggested is not as common in Europe as the USA.
  This is also why I said I'd ask for their biological sex, since OP asked to know about man vs. woman.
• The person is not an outspoken LGBT activist, so they might not even consider there to be ambiguity regarding their sex, causing surprise/embarrassment.
• OP seems a bit shy in approaching the topic.
• OP is comfortable to make out with the partner without knowing their sex/gender identity.
• OP stated,the result wouldn't really matter either way.
• OP seems to be bisexual, ergo either sex would be fine.

Anything else seems sneaky and dishonest (you may try some of the suggestions here nontheless if you like).

As a european I'm in agreement btw. with ApertureSecurity about the different approach and self evident understanding in comparison to american society.

Non binary pronouns (them,they,ze etc.) for example are not even a noteworthy topic (and are quite a bit more problematic to implement) especially in non english speaking countries here, since many languages have male and female ingrained into them in prefixes or suffixes for example in addition to the pronouns to address the person.
Some transgender people chose the gender neutral pronoun "it" surprisingly, despite being a bit strange to refer to a person as "it".

• Asking flat out about gender identity/pronoun as was suggested is not as common in Europe as the USA.
  This is also why I said I'd ask for their biological sex, since OP asked to know about man vs woman.
• The person is not outspoken LGBT activist, so they might not even consider there to be ambiguity regarding their sex,causing surprise/embarrassment.
• OP seems a bit shy in approaching the topic.
• OP is comfortable to make out with the partner without knowing their sex/gender identiy.
• OP stated,the result wouldn't really matter either way.
• OP seems to be bisexual,ergo either sex would be fine.

Anything else seems sneaky and dishonest (you may try some of the suggestions here nonetheless if you like).

As a European I'm in agreement btw. with ApertureSecurity about the different approach and self evident understanding in comparison to American society.

Non binary pronouns (them,they,ze etc.) for example are not even a noteworthy topic (and are quite a bit more problematic to implement) especially in non English speaking countries here, since many languages have male and female ingrained into them in prefixes or suffixes for example in addition to the pronouns to address the person.
Some transgender people chose the gender neutral pronoun "it" surprisingly, despite being a bit strange to refer to a person as "it".

• Asking flat out about gender identity/pronoun as was suggested is not as common in Europe as the USA.
  This is also why I said I'd ask for their biological sex, since OP asked to know about man vs woman.
• The person is not outspoken LGBT activist, so they might not even consider there to be ambiguity regarding their sex,causing surprise/embarrassment.
• OP seems a bit shy in approaching the topic.
• OP is comfortable to make out with the partner without knowing their sex/gender identity.
• OP stated,the result wouldn't really matter either way.
• OP seems to be bisexual,ergo either sex would be fine.

This question is quite similar to not remembering a persons name after interacting with them for too long to still be asking for their name.

(Note: please see my summary at the end of my answer, I don't have time at the moment to rewrite to fix apparent lack of understanding of what I'm trying to convey.
Instead of downvoting I'd appreciate a comment suggesting improvements!)

This question is quite similar to not remembering a persons name after interacting with them for too long to still be asking for their name.