i see your point... when you are gay or straight and you're dating someone who is bisexual you have twice the competition... but, it probably wouldn't bother me because i look for guys who don't think with their cocks. usually, if you're with someone who isn't lusty and promiscuous, you'll feel more secure around them and about your relationship with them.
...so basically, it wouldn't matter just how much competition you had, because you would trust eachother more, and thus be more compelled to eachother.