Sex Drive

So my partner and I usually have a great sex life. When we first started having sex it was an almost daily thing. Now we are down to twice a week about, always good sex where both of us achieve orgasm.
Now he's asking that we not be as frequent. He has a lower sex drive than me. He says every time we see each other we have sex and that's true, but I feel like he's not factoring in  ages (18 and 20) and he's not factoring in that we see each other for maybe a night and half of a day once a week or every two weeks because he works nights and I'm a full time student. 
I'm unsure how to react to him wanting less. It's very frustrating to be teased and promised sex and then him bring up that I seem to expect sex every time. I feel like it's a crime to be sexual. He's my first sexual partner and I want him all the time even though I've been having sex with him for close to four months (dating for nearly five). 
What do you do? Do you buy a vibrator? I've tried talking to him about it but I get mixed signals. I also am one of the girls who gets lighter/ less painful periods if he has sex with me before and during periods, so it's not just sexual frustration I'm dealing with here....