Okay, this is just my opinon, but I think it was just a "phase" he was going through. No, not bisexuality, but during the Pop times he said and did some things he wouldn't have done before or since. He seemed (to me) to be trying so hard to be different that he was going out of his way to destroy his previous rep, his image and the way people thought of him. To be honest, I wasn't pleased at all, since I had loved him deeply for all that he was, and he had become, so, well, different. So this was just one of those actions he thought was cool at the time, but would likely feel different about today. He pecked Bowie on the cheek last year, but that was just cutesy, not this exaggerated tongue crap. Personally, IMHO, I think the pick is disgusting, but again I wouldn't want to see anybody, gay, straight, or bi, do that in an up close pic either. Yeah, I think they were joking and going for shock value, nothing more. Bono and Liam are both very heterosexual. (not that it matters if they weren't of course!)
BTW, Bowie is straight, too. He admitted a couple years back that he was never really bi, he was only acting like it to shock people and get attention. It was part of his act. He said yes he did experiment but didn't like it and he is very straight and wants women exclusively. (not that it would make any difference if he were bi, just setting the record 'straight'
)
Bono and Liam are now just good friends, as you can see here.
though it still doesn't explain why he picked that awefull Galleger (sp?) guy to kiss 
This is just my opinion of course. 
Some people are just like that.
sounds reasonable and all
