I guess that many of us, bi-curious guys are sharing your story and debate the same communication challenges with our female partner.
I think that there are two major gaps in how the opposite gender perceives our curiosity:
1. Traditional social conventions perceive such male as a none-alpha-male; a sissy that contradicts the conventional functions of a patriarch.
2. Women cannot share the heavenly pleasure that we derive from another cock in our ass that massages our prostate gland.
We should go for it nonetheless or die with a strong sense of regret
I personally have given up on convincing my female partner that bi-male activity is fun and Kosher; the very same partner that loves double-penetration and lesbian pleasures
Luckily I have other alternative to meet one or two male friends of mine, such as in my office or one of our flats.
I don't have enough words to express the difference in sexual pleasure, among which is the freedom of expression, and unisex understanding and communication.
Also - fucking another guy or being fucked by him doesn't sit on my conscious as if it would be cheating with another woman.
Go for it my friend, and I'll be by your side to hold your balls and twist your nipples for as long as you like.