The way to become bisexual is to regularly extend your exposure to erotic stimuli just a little further than your comfort zone extends in that direction. I'll use drawn pictorial porn as an example erotic stimulus, but adapt to whatever you prefer: start with Bridget. Everyone is gay for Bridget. Once you're comfortable with Bridget, move on to futanari-on-female erotica, male-on-futanari, then futanari-on-male, paying attention to your comfort levels. You'll run across some bizarre things while searching for this stuff; if any of it interests you, just go with it.

By now, you should be fairly comfortable with the plumbing involved, so it's just the somatically male body you need to learn to find attractive. Find art featuring bishounen types, then pairing them with other male body types, and pay attention to what feels most comfortable.

It may take a while to go through this process, but I believe it's entirely achievable for most people who don't view heterosexuality as a terminal value.