I don't know, RockRose, goody-two-shoes can be really hot. ;-)
But seriously, daddysgirl, it's a much more common thing than you think... and probably has nothing to do with you at all. For instance, if he's used to masturbation then he might not be quite adjusted to sex. It's very likely. Also, there could be some physiological issues going on: sometimes if one is really excited, then something happens and one can lose one's erection... it's hard to explain, and frustrating as hell, but it really happens. Or, as Marra315 said, it could be performance anxiety. Maybe a combination of all of the above.
But the bottom line is, if he has an erection during the introductory make-out & foreplay, then it's DEFINITELY not you... trust me! It makes alot of sense to me, it's just hard to explain. Soemtimes when one (a guy, that is) gets excited, even if he's really horny he may still lose the erction. Trust me, it's happened enough times, and it's a conundrum and a nuisance, but it's nothing to worry about. Just take it easy, and tell him to do the same.
Why do you call yourself "daddysgirl"?
Maybe some clues there.
What kind of sex are you doing when you say: "we start having sex"? From my perspective, some women have one thing on their mind when it comes to sex, and men may have another one or two. So, you are not the problem, since your boyfriend is getting turned on by you, but perhaps the type of sex that is going on is not what your boyfriend wants to carry out, leading to lack of interest. Being more direct, would he rather be doing oral, anal, 69, analingus, you on top, in the living room, in the car, outdoors, handjob, and so on? Of course, any outside stress influences would affect the situation and create a distraction. I hope this helps.
i dont think its you , i think he has psychological issues . is he stressed ? how long you been together ? could be performance pressure !