Absolutely, I don't think so far any of my spouses has been loyal and I always make them at least my lover. A particular one comes to mind that was soulmates with my ruler at the time and yet I got message after message of children's heritage being disputed. At least five of our children turned out to be someone else's.
The game, as it is, is massively oversexed especially given the historical context. Please keep in mind that the "sexual liberation" and promiscuity is a very modern thing, back then adultery not only was a huge taboo, it usually was very drastically punished as well. Plus, in a game that's all about family lineage it's not exactly fun to learn that your heir actually was never yours and thus the family line has been broken, and this happens with such a ridiculous frequency...
At the VERY least the chance of a spouse accepting a seduction attempt while they're in a lover or even soulmate relationship with the one they're married to needs a massive hit. When soulmates, it should be downright impossible for them to cheat at all. I don't have the game open right now but I remember the description was something like "a very profound, deep romantic connection ... you can only have one soulmate at a time". Yet they'll then go about cheating at the drop of a hat. How does that make sense?