I read an article once about how primates have oral sex before actual intercourse. IIRC, it claimed that arousal before intercourse increased the chance the sex would result in pregnancy. Would look for it now, but it's kinda NSFW.
But it could just be that the ability to suck is important for other evolutionary reasons, and the ability to suck cock just came along with that.
Not only arousal, you can pretty easily find some papers on the 'net talking about how it increases the chance of a successful pregnancy as it allows the immune system to adjust better to recognising the foreign matter as friendly.
Name:
Anonymous2010-04-04 5:03
There is minimal or none selective pression for fellatio. We like it because we like having the pënis stimulated.
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Anonymous2010-04-05 11:08
>>7
I think you need your pënis stimulated before that last statement can be considered accurate or not.