It is totally an attention thing... I may have been known to do it once or twice in my lower-self esteem days (although I never kissed my girl friends - just lots of dancing and flirting). Guys ate it up...
Also, it is a money maker. I know lots of friends who have made out in front of random drunk guys who offered them free drinks and/or cash.
If that's not one of their motives, then I guess they really are bi-curious or bi.
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