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when your friend is going out with another person, and they invite you along for the sole purpose of making sure the third person doesn't think it's a date.

in effect, your friend asks you to come along, just to defuse the sexual tension and make sure nothing happens without them having to verbally reject the other party.

note: Sex Buffers work best when you and the third party are of the same gender so they don't try to invite a 4th person to make this into a Forced Double Date
Guy 1: hey what did you do last night?
Guy 2: i went to the bar with that girl i was telling you about...
Guy 1: niiiice. did you hit it?
Guy 2: no! she brought her roommate as a sex buffer. he kept getting in between us and cockblocking me.
by Shaggo August 19, 2011
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