Normally when my Sims find someone that they're interested in, I trial it out with three dates. If all three dates are successful and everyone seems to be having a good time, then they take it further. If not; they both move on and try again with someone else.
On an old save, I had a random event happen where the woman asked her husband whether or not he'd agree to an open relationship (I was using the Slice of Life mod at the time), he said no and they ended up divorcing. She then got with someone else and had a child by him before even trying the three dates thing. So sometimes my games don't always work out the way I want them to, but it keeps things interesting.
