This is actually something that has happened to me before. A female coworker came to me asking if I could help her learn some skills that would advance her career. I agreed to teach her and she ended up making romantic advancements toward me. I immediately told my significant other what was happening and started working towards resolving the issue.
Agreed. I had a free ticket to a comedy show, and I wanted to give it away without it going to waste - I had to think very hard and carefully about if I wanted to offer it to a female coworker on short notice.
The comedian in question did some rather blue jokes, and that could possibly be construed as harassment. It's a shame you can't just offer tickets to coworkers of the opposite sex without worrying about consequences.
The incident made me fairly wary.