I strongly disagree that "acting in passion" in any way mitigates murder or battery. In the first place, I notice that such crimes tend to be committed by people with a history of abusing their partners. In the second place, I think such crimes are based on the groundless expectation that we somehow possess or own our partners. Lastly, making exceptions for "acts done in passion" sends the message that rage makes murder and battery more socially and legally tolerable, and thus encourages people to act in rage.
If you want a monogamous partner, then look with great care at your potential mate's proclivity for monogamy, and on their ability to keep promises, but don't merely assume that s/he will be faithful simply because s/he's in a relationship with you, or even simply because s/he loves you. Neither a willingness to engage in a relationship, nor even love, guarantees someone will be monogamous. Again, a partner that must be coerced by laws, guilt, or social pressure into monogamy is likely to find ways around those things when tempted. Better to find someone who wants to be monogamous, rather than find someone you think you can train to be monogamous.
