If you're really serious about this anon, the best thing I can think of is to ask her if she'd be interested in an open relationship since you shouldn't be a piece of shit and do it behind her back, or at the very least just once to try it. However, a word of warning: you need to be prepared for the worst case scenario if you ask that question since it's a very big thing and could possibly impact the rest of your relationship with her, which is her not being ok with it at all and breaking up with you for whatever reason she may have. Not trying to scare you or sound irrational or anything but there are plenty of straight women out there who would never touch a bi guy because of the stigma that we're all promiscuous thus much more prone to STD's, we'll never be able to truly love a girl because we'll always want guys too, etc. I'm sure that that sounds stupid or even entirely unrealistic but trust me, there are plenty of women out there who think this way, my own mom being one of them. Whatever you end up choosing, I wish the both of you the best and hope it goes well. Good luck, anon. :)