Bro, I understand how you feel coz I am same age as you and sometimes I have the same problem.

I would suggest that you need to exercise regularly at least twice a week, jogging and most importantly - STRETCH your bottom part will be a great help. I understand people these days sit for a long hours, so am I. So I suspect its poor blood circulation. Because whenever I skip exercise for that week, I will face that problem.

If you do not want to try any drugs. You can try eat oysters a day before that. There is no solid prove from Science, but it works for me. Oysters and crab is one of the highest source in Zinc, and Zinc improve sex drive.
http://voices.yahoo.com/do-oysters-increas...ve-1007085.html
http://voices.yahoo.com/zinc-healthy-amoun...4301.html?cat=5

Do give all these a try before you go to consult an urologist or physiologist, basically they will just tell you its your mental, just relax and try to listen more relax music.

Also, do not drink alcohol too often

So hope this helps, and look forward to your great news about she enjoy it.