Definitely Less than Zero. One of my favorite 'brat pack' movies, although it was on the fringes. I also kind of consider it a semi-Christmas movie, since it takes place at Christmas.
Robert Downey Jr's character, Julian, is heavily in debt to his drug dealer (played by James Spader at his sleaziest), so Spader's character makes Julian turn tricks to make up some of the money that's owed. Spader also gets Julian back on the drugs, which leads to Andrew McCarthy's character trying to find Julian, hence the scene where he walk in on the bj's/sex.

You're welcome. Glad I could help.

Thank you. That sounds probably right.
Thought it was Downey Jr, but just watched St Elmos Fire and vaguely remembered that scene so confused with Lowe.

Less Than Zero (1987)
Robert Downey Jr