First off, it is very common in this lifestyle, for all sorts of reasons.....none of which have to do with how hot your wife is, so tell her to relax about that, I'm sure she is very sexy
Most of the time it has to do with performance anxiety and once it gets in the head it is hard to get out.....alot of men, my husband being one will jsut take it slower and hang with me, but since the single guy does not have that to fall back on......sometimes it helps for you all to go a little slower and have more caressing, foreplay time, not necassarily oral because sometimes that is a stressor as well because all parties involved know what we are all waiting on........I hope this helps some I am not an expert and Im sure that you will get alot of good responses here, but I just wanted to reasure you that it is pretty common and it is def NOT your wife.