When two people wanna have sex but can't, for a variety of reasons - already in a long-term relationship; it would look improper to date a co-worker; or simply, they're both too shy.
"Hey there's a lot of sexual tension between Gus and Wendy."
"Yeah, tension's all. Gus is married and Wendy's his boss."
The scene in captain america:the winter soldier where Steve Rogers pushes Natasha Romanoff against the wall in the hospital. Or the car scene after the escalator scene or the scene in Sam’s house where the two of them are talking about trust.