After more than 3 years of sex with my girl, i'm not fully satisfied about the penetration itself. I mean the act of penetrate - once inside it's absolutely heaven! I need to put lot of saliva on the tip of my penis, even though she's very well lubricated. I notice that when I'm ready to penetrate her, she is very nervous and I guess that she gets stiffen, maybe for the fear of feeling pain. How to relax her? PS: I want to add that she has very, very small inner labia, don't know if maybe this can be a problem, perhaps because penis can't be accommodated in a proper way. Dunno!
Sometimes If I don't use extra lube on a condom during sex, when she starts having an orgasm, when she tenses up, sometimes it just makes my penis push back out out of her. That's when extra lube is a bonus as it prevents the penis being forced back when she contracts on it.
It's no secret that some girls are naturally very small and tight. Just take it slow. Use heap of lube - maybe give her an orgasm before you penetrate. (tongue or fingers)
Before going for the penetration, keep stimulating her clitoris with the head and / or the underside of your penis. Not only will this give her additional pleasure, it will feel pretty good for you, but it will also tend to relax & lubricate her in readiness for penetration.
I already give her an orgasm before penetration, rarely I penetrate her without foreplay (it depends on time we have). I think she's a little bit tight, actually.
OP, I can relate in that my wife (contrary to previous GF's) is a bit extra sensitive down there. If there's no lube or not a lot of foreplay then it's closed for business, case in point morning sex. When there is lube / foreplay I still have to take that first entry slow and gentle. Once I'm in I can jack hammer away. This is why we keep a bottle of lube by the bedside And saliva is a bit overrated. It may seem like lube and can somewhat help out, but it's like saying water is a lube. Just because it's wet doesn't make it a lube. So do you and her a favor and GET SOME LUBE!!!
I would guess the reasons are two-fold. They are non-monogamous and that keeps them safer, and at their age, they definitely don't want a baby. I've been married five years and we still use condoms on the rare occasions we have sex.