Okay, so a little context. My (22m) girlfriend (21f) doesnt seem to have sexual feelings like i do. We've been together a little shy of three years, and our sex life has always been questionable at best. Neither of us have had other sexual partners, and we are each others firsts. We used to have sex 2-3 times a week, but then that went down to 1-2 times per week about a year ago. Then we went long distance and now that we are back, it feels nonexistent.
Now when we do have sex, it is pretty good. But the problem is she only wants to go for five minutes and be done. I've tried to verbalize my concerns, but i always get met with, "Well we already do it a lot, you should just be happy with what we do." and that kinda makes me feel invalidated. I don't ever get to get off, she never advances me, and she doesn't seem like she is interested. For me, sex has become a decently big portion of a relationship. If we don't have sexual chemistry, the how does she expect to have kids. (we have talked about wanting children). So my main question is this; how can i address my needs without sounding like i'm needy or ungrateful for what we have?