You don't have to have sex every single time that your boyfriend claims he's in the mood for it. There's nothing wrong with turning him down. He might be upset about it for a minute or two, but he'll eventually get over it. If he doesn't, then he's not the right one for you. Remember that these are a few perfectly acceptable reasons for turning down sex:
1. You’re Not in the Mood
You're not going to be horny 24/7. It's impossible, so don't feel bad if you aren't in the mood at the same time as your boyfriend. You don't owe him anything. Even if you two made out a bit and you gave him blue balls, it's no big deal. If he's upset about it, then he can go into the bathroom and take care of the problem with his own two hands.