Bonus: These are all adult, but in YA, try I’ll Be the One by Lyla Lee. Also, while not a Romance by genre definition, Verona Comics by Jennifer Dugan has a central m/f pairing and both are bi/pan
Double Bonus: While the (male) MC of The Girl Next Door by Amy Jo Cousins does not ID as bi (though the female love interest does), it would be safe to call him “heteroflexible”