I've had this happen once I think. It was with my ex when we hadn't had sex in a long time (had been 'apart' for about four to six months or so). Anyway she was so horny I just went right in. Which was hot and unexpected because she was a virgin when I met her and had some trouble at first, I thought she would have tightened back up. But she loved it and I think she just arched her back a certain way and took me to the hilt.
I wouldn't worry about it. If a girl is really turned on then she should have no trouble taking a cock your size. Mostly when there is a problem its because the girl hasn't been turned on enough.
Of course if she's a virgin or inexperienced there might be a bit of discomfort. But I would take it as a compliment/turn on that a girl took me all the way inside on the first push.