Guys; why would you not want to sleep with your girlfriend?

My boyfriend and I have dated for about a year and a half and I’m his first long term girlfriend.
he has always been shy and not as interested in the sexual side of stuff, but I am more into it and need it more than he does.
Slowly he has become more into it, we are in a long distance relationship due to work so we only see each other once a week, and this means he is quite tired/or we are both tired which affects our time when we do try to sleep together.
He tends to only be able to go once whereas I’m able to go a few times, but the last time he wasn’t really into it at all, I noticed it when I saw him he didn’t kiss me like normal and then he only went once but he always makes sure I’m happy, but this time he didn’t seem into it as usual... I want him to want me more like he doesn’t appreciate me or finds me sexy enough?
Guys; why would you not want to sleep with your girlfriend?
Post Opinion