I play many varied families -- gay, straight -- with bisexuals added into the mix. Many couples have children, but some do not. I have one extended gay family which using the same-sex pregnancy option I've stretched reality a bit and made them a multi-generations "legacy" family of sorts. Generally pick one child to carry on the family and the others sort of mix and match according to their own interests. Lots of straight families as well.
My sims on the whole tended to stay monogamous, too. But now that Nightlife has come along the whole dating and relationship thing has changed and here I am in a neighborhood full of already paired up couples for the most part!
It's made me re-think my carefully laid plans.
I've got a male couple where one is bi (Logan) and he's ready to jump ship with the single girl who lives across the street (Rachel). They met in college. She's a romance sim, and he went from popularity to pleasure sim. The chemistry between him and his boyfriend (Scott) is fine, but she outscores Scott by a huge amount on the LTR for Logan.
Then I've got this straight couple, Todd and Roxanna. They just got married and she got pregnant before I installed Nightlife. I thought of them as "made for each other", and they are just as sweet as anything with 100/100 relationship scores for each other. And they both come from long-run families in my game. So then I install Nightlife, set their turn-ons and turn-offs to compliment each other... and they only have "weak" chemistry. One lightning bolt.
After she has her baby, I have them try for another. Two attempts and not only no baby chime but their woo-hooing has no fireworks! I then forced the pregnancy to happen with a cheat, and she gave birth to a baby boy.
One other gay couple (from the legacy family) also seemed to have a pretty happy marriage with one kid already. Then while I'm playing a single gay male (Leo, a family sim) going out on the town for dating, he meets the half of the couple who's also a family sim (Charlie). They have three lightning bolts, and after some mind interaction with each other Charlie keeps showing up at Leo's house and ringing the doorbell or calling and asking to go out. Now I was very reluctant to indulge this and trying to figure out why would a married family sim behave this way? So I kinda let things develop organically. Leo got into a big altercation in front of his house with the local mean-guy sim who kicked over his trash can. Afterward he called Charlie and they met at the 50's Diner. They had a "Dream Date" and even though they went back to Leo's, Leo wouldn't have him come inside. They made out by the front door and Charlie went home. Then Leo got roses from him... (sigh)
In all my families, I develop back-stories to fill in the "why" questions for things happening the way they do.
Logan is simply a flake. He's an broadway star actor, stunningly handsome, and a playboy. He's already pissed off Scott with his shenanigans with Rachel, and Rachel is just aching to put a Logan-notch on her bedpost. It remains yet to be seen if Logan and Rachel just have a fling or stay together, but Scott (a really sweet guy in many respects) is just going to have to join the single crowd again.
Todd and Roxanna are going to find that the initial pizzazz they thought they had is just not deep enough. They'll likely carry on for awhile but I think times ahead are going to be rough for them. Sometimes the best laid plans just don't work out, just like in real life.
Charlie and his husband Brandon are an interesting case. They only have medium chemistry. Brandon is kind of a odd guy, but likable. I can only interperet Charlie's affair as a deep-seated dissatisfaction with Brandon despite the fact that they have *another* kid on the way. I feel fairly certain they will break up and Charlie will gravitate to Leo. Who gets the kids will be interesting. One of the kids will be next in line to carry on my legacy gay-family's next generation. Brandon, I'm thinking, would be the perfect candidate to let this experience nurse his bitterness and raise a cowplant since I haven't had one in my game yet. I'm not going to let it eat Charlie or Leo, but I keep thinking Brandon might turn psychotic and feed some townie dates to it. He really doesn't have chemistry so far with anyone except Charlie for some reason.