Study takes 50 men and 50 women who have had sex (not necessarily with each other) and asks them what their reasons were for doing it.
Here's the PDF of the paper with the full list, which is actually way more than 50 reasons.
Here are a couple of goodies:
The person was too hot to resist
I wanted to say "I've missed you"
The person was intelligent
I was drunk
I wanted to get a job
I wanted to feel closer to God
I wanted to hurt an enemy
I wanted to get rid of a headache
I wanted to end the relationship
Definitely some worth keeping, should you ever require an excuse.