I applaud you for wanting to be better.

Try thinking about his potential attraction to other women. Do you feel threatened if he finds another woman in public attractive? Do you trust him not to cheat on you with other women in his life? If you don't feel threatened by other women and you trust him, then you shouldn't feel threatened by his potential attraction to other men.

You may say "but I don't have a penis" and worry you can't satisfy him. To that I say it doesn't matter. He has given up other attributes in women that you don't have because he wants to be with you and he doesn't think twice about what he's giving up. Other women might have bigger boobs, are more fit, can cook better, are more adventurous, have a better sense of humor, prefer different sexual acts, but he doesn't care because he wants you. It's the same with other men.

If you read about bisexual people wanting to cheat, it's not because they are bisexual, it's because they are cheaters and they come in every sexuality. Many bisexuals have zero issue staying monogamous.

Another thing that might help is really diving in and being vulnerable and open about what you each find attractive in different genders. Maybe he would do everything with a man or maybe he just wants to be held by someone stronger. Maybe he wants to go down on a guy or maybe he just wants to bottom. Maybe he wants to make out with another guy or maybe he just wants to touch his defined chest. Understanding this might help you.

Last thought...you don't need to do this, but a partner who is willing to be adventurous sexually is sometimes really appreciated. Try some new toys or role play. Maybe he'd like to be pegged or be the vulnerable one that gets gently dominated. Have fun and explore with each other.