Nobody is asking you to find M/M attractive. That would be silly: I don't find F/F or F/M attractive and I think I'm mostly not a bigot. The issue is that, if I've read your post correctly, your specifically have a problem with characters being bisexual, which strikes me as problematic. Bisexual people are just as capable as straight people of being loyal to their partners. You never have to think about a bisexual man with men if you don't want (just like you wouldn't have to think of your straight partners being with other women.)
Biphobia is a pretty real problem that most people are pretty dismissive about; I used to pretty regularly read a gay forum and it was disturbing to see how many people "Wouldn't want to date a bisexual". While I'm happy with "set sexualities" (I put that in quotes because I choose to believe that sexuality was set in DA2, they just all happened to be bi) I still find it disturbing that I feel like many of the complaints about DA2 come from this biphobic stance. I've argued with people who think that there was no way Fenris or Merrill could be bi because "it didn't make sense for them" like it did for more promiscuous characters.
If you have lots of gay and bisexual friends then you're probably a pretty open person when it comes to sexuality, and I'd encourage you to reevaluate why you're uncomfortable with the idea of a LI being bisexual.
Biphobia is a huge problem in both the straight and gay communities. I work at an LGBT center and I've even heard volunteers say incredibly offensive things about bisexuals. Gay men that are just too afraid to come all the way out of the closet. Straight women that just want men's attention. Sluts that will bang anything, or that can't decide what they want. It's really upsetting and I've gotten into some heated arguments with people about this in real life.
As a general rule, I think people should consider that, if similar things said about you would make you offended, don't say them about bisexuals. If you wouldn't appreciate being told you're "confused" about your sexuality, whether you're gay or straight, then why would it be acceptable to assume you know someone else's better than they do? If you don't think it's alright for people to assume you will sleep with any man or woman just because your sexual orientation lends to them, don't assume bisexuals will sleep with anyone just because they can potentially be attracted to either. If you've ever been attracted to more than one person in your life, I doubt you'd want someone telling you that you can't "make up your mind" about what you want. I think we see the pattern here.
And further on that point, if you would be alright with hearing about someone's partners or exploits if they are one sexuality, then I really do think you have some issues you should look at if you do have a problem hearing the same thing from another sexuality. Double standards are generally always a red flag that something is not right with your outlook.