Is it normal that i become turned off by my boyfriends over time?

I have been with my boyfriend for over 3 years. There are some minor issues in our relationship, but overall, it's pretty good. But I am completely not attracted to him sexually. I think he's cute, but when it comes to sex, I am totally unaroused. It wasn't always like this. In the first few months, I always got turned on when we were making out etc. But after a little under a year...nothing. 3 years later, we still have sex frequently, but all I care about is making him feel good. He has no idea I'm not into it and I just couldn't break his heart by telling him.

Now you might say it's normal for the spark to die. But it's more than that. I am actively turned OFF at the thought of him. If I'm masturbating and think of him, I won't be able to orgasm. Now you might think I must be grossed out by him. But I don't feel revulsion during sex with him. Just zero arousal and a mild desire for it to be over.

And here's the thing...this has happened with all of my boyfriends. It's not just him. Now granted, I've only been with 2 other men, and neither was what I'd call my "ideal." But I was turned on in the beginning. However, after several months I was turned off at the very thought of them (not a little bored, not grossed out...just plain turned off). I'm beginning to think this is "me."

Here are the important facts:
1) I'm a 22-year-old female.
2) I have trouble having orgasms during sex, but not when I'm alone.
3) I'm not a lesbian. I do experience attraction to males, it just fades.
4) My boyfriend has gained a lot of weight. However, I became turned off by him before this happened.
5) My boyfriend has a slightly below average sized penis and some performance issues. However, this has always been so and didn't used to turn me off.
6) None of my boyfriends have been extremely attractive to me, but I was attracted enough to them in the beginning.
7) I have a fairly high sex drive and often masturbate and fantasize...but sadly, only about other men (and occasionally women, but see #3).
8) I'm rather inexperienced with relationships, having only had 3 serious boyfriends and sexual partners including the current one. However, I've also had several casual relationships where we did not have sex...and my attraction to them faded too despite the fact that we never had sex.

You might say "just break up with him," but I'm afraid this would happen in any of my future relationships.

So to sum up this tl;dr question, is it normal for some females to chronically become turned off by their boyfriends? And what can I do about it?