While it's only happened to me a few times, I've heard that many guys have a problem with keeping it hard when they have to break to put the cover on. Whether it's because of the lull in the time or because the condom is too tight, losing one's hardness can be a real drag. The following link has some great suggestions for keeping it hard while practicing safe sex:
http://www.naughtyreviews.com/forum/hobbyist-hangout/going-flaccid-when-the-cover-goes-on
http://www.naughtyreviews.com/forum/hobbyist-hangout/going-flaccid-when-the-cover-goes-on
