Um, wow. Okay first thing, understand you are making a TON of assumptions that are just not good. Assumptions really don't work out well in poly. Clarification does.
Now, to the stuff you are assuming. Straight women have threesomes. So just because a woman and a man are straight does not mean no threesomes. SO there's that assumption blown out of the water.
As for straight couples being poly and not able to be fully integrated, another assumption. My husband, straight, does that mean he can't REALLY be part of my relationship with my boyfriend? Um well, it wouldn't matter either way. It's MY relationship with my boyfriend so would prefer to NOT have them be interested in a threesome. Nothing wrong with a threesome but guess what? Not required!
As for HAVING to date on their own, yet another assumption. While it seems to be awfully popular for new poly couples to want to want to date together, many don't want to! Hubby wants to date, he can date without me. I date, I date without him.
So really, think about what the real issue is here. Do YOU want to be dating a couple? You are welcome to, go for it! However, if you are dating a man, and not his wife, then that's the situation. If she isnt' interested, is that a problem for you?
What I'm trying to say is there is, for many people, nothing to get around. I'm bi, but I date one person at a time. So the fact that I'm bi doesn't mean that my husband or my boyfriend are going to get a threesome. Or that they have that opportunity with me. Lots of assumptions. Try clarifying what you really want or are worried about and then clarifying with the husband and wife what they are wanting or willing to have in a relationship.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year