If it is a first contact for both of the partners then there might be a problem with insertion of penis, breaking of hymen. It can be a reason for loss of erection, foreskin traumas; also partner might start to bleed. That of course can cause the unpleasant feeling for man.
In cases if development of foreskin has been problematic, with erection, when penis bulges, it might bring some pain and consequently a loss of erection.
If woman has very narrow uterus, man might get bruises on his foreskin. Consequence is red and burning foreskin. Also foreskin ligament can break. In this case you can feel sharp unpleasant pain right away. As well it is possible to have a broken penis – it means broken tissue and bruise. Of course it happens very seldom and reason is very brutal or rough sex. If it happens, shape of penis changes and you have to turn to doctor as soon as possible. It is healable and if you turn for help soon enough you might not have any unpleasant consequences. Of course, the owners of big and wide penises should remember that inserting them into their partner might bring some unpleasant feelings. Remember that during the sex you have to think of not only yours but also partner’s pleasure.