Hello Leer and
to GaySpeak. I have an idea here that might be what's causing it, but maybe not. I don't know if someone else has suggested it here either, as I've not read everyone's answer.
I hope it is a new way of looking at the problem.
I'm not sure there is a problem with sleeping with a man rather than a woman, but it is probable that you can't "shut down" on account of heat. When you are lying in bed with just yourself, your body shuts down and gradually lowers your body temperature (and heart rate) so it is possible that your body, lying next to another human being's body can't get cool enough to shut down. I think it would be worth your while trying to sleep in some other accomodation, be it a mattress on the floor or a sofa if you have one. I have noticed having trouble sometimes falling asleep myself when my partner's body heat is too high for me to shut off. The bed is wide enough that we can generally find our own space in it, I try taking the covers off me till I cool down enough to feel drowsy, but he likes to have the covers snuggly around him. When I feel I'm ready to shut down, I will cover my body up again and lie next to him, but by then he's shut his body off too and his body temperature is surely lower too. The fact of having sex beforehand might possibly heighten both your body temperatures a bit and pace your heart rate faster, so it could be that you're still in a state of mental excitement which does not help to go to sleep. Most people would find that having an orgasm shuts their body down quicker as the endorphins kick in after ejaculation, but it is possible that it has a reverse effect on you.
Quite honestly, if you are having trouble sleeping next to someone (man or woman, doesn't matter) I'd go and discuss it with my doctor. GPs are likely to know what sort of questions to ask you and are more likely to know about sleeping troubles than we, ordinary citizens, are. It's worth finding out, even if you are a light sleeper, just in case it starts affecting your life and health at some point.
The other thing to check out is whether you're giving yourself enough time to wind down at the end of the day. Staying online, for example, or playing video games till late will give your body ( and brain mostly) the wrong kind of stimulation, so it might be that you are subjecting yourself to all sorts of different stimuli, that, in conjunction, are creating the problem.
Even though you say you are a light sleeper, there must be a problem, otherwise you wouldn't be posting here about it. See what your doctor says, that's what I advise you to do, or else, try a cooling shower before you lie down again. If all else fails, you need to sleep
alone. Your doctor would know statistics on how common or rare that condition is, but it doesn't sound like you need to worry too much.
It will be a bit of a sensitive issue, maybe, with a partner, but if they care for you enough, they'll know that sleeping alone is what will keep the relationship going strong.