• Rather than explaining the preference, explain the framework.

I made the experience of dating two people at the same time. The two most stressful weeks of my whole life. It's not that I couldn't; I'm just bad at multitasking in a more general way and I need some consistent time alone.

In your question, you write something pretty similar about preserving energy. You could introduce the features of your personality that lead you to such a preference before talking about the preference itself. After all, your preference doesn't come out of the blue; on the contrary, it is the product of your personality, needs, points of view etc. Talk about those. It is a matter of both dropping phrases here and there ("I'm too stressed out by multitasking, I feel like I have to finish a thing before starting another") and talking thoroughly about a topic, for instance being an introvert that needs some time alone to recharge his energy or something alike.

• Show them that you have a social life.

Mention your friends to them, tell them what you did last night with your flatmates, send a picture of something a friend will appreciate in their presence and explain it to them. In short, show them that they are not the only person in your emotional life and that you don't need them to have an emotional imbalance. Of course you'll be more than happy to be with them, but if they can see that you're an emotionally mature person they will feel freed from the weight of being the older guy.

If a) the explanation of your monogamy comes after some unrelated disclosure of your personality to them; and b) they can see they're only a part of your life and not the center, by the time you'll tell them that you prefer to date one persone at a time they will know that it is a matter that fits naturally in your personality and that it's not related to some ill attachment to them.

• Rather than explaining the preference, explain the framework.

I made the experience of dating two people at the same time. The two most stressful weeks of my whole life. It's not that I couldn't; I'm just bad at multitasking in a more general way and I need some consistent time alone.

In your question, you write something pretty similar about preserving energy. You could introduce the features of your personality that lead you to such a preference before talking about the preference itself. After all, your preference doesn't come out of the blue; on the contrary, it is the product of your personality, needs, points of view etc. Talk about those. It is a matter of both dropping phrases here and there ("I'm too stressed out by multitasking, I feel like I have to finish a thing before starting another") and talking thoroughly about a topic, for instance being an introvert that needs some time alone to recharge his energy or something alike.

• Show them that you have a social life.

Mention your friends to them, tell them what you did last night with your flatmates, send a picture of something a friend will appreciate in their presence and explain it to them. In short, show them that they are not the only person in your emotional life and that you don't need them to have an emotional balance. Of course you'll be more than happy to be with them, but if they can see that you're an emotionally mature person they will feel freed from the weight of being the older guy.

If a) the explanation of your monogamy comes after some unrelated disclosure of your personality to them; and b) they can see they're only a part of your life and not the center, by the time you'll tell them that you prefer to date one persone at a time they will know that it is a matter that fits naturally in your personality and that it's not related to some ill attachment to them.