Why did it turn him on?

Right well i have this friend (i wont say her name) but basically we're just like sisters and we're always together, and now she has this boyfriend, he treats her right and their really happy together, but anyways,we were at hers and we kissed as in like a sisterly we, on the lips but on a little peck and we do it as a laugh (we're both completly straight its just in a little loving way) but he was like oh yehh thats boss that we were all alughing then the morning after we both wanted to wear this top so we were both topless trying to get this top and we both landed on the bed she was on top tickling me trying to get this top (Im 14 and she is 15, 9 months between us) and he was like getting turned on and saying this was the best weekend ever? why did it turn him on? why do girls on girls turn on other lads? does it turn on all lads (apart from homosexuals)? haha xxxxxx sorry but just wondering lmaoo:D