Some people find monogamy genuinely difficult, and unnecessary and stressful, and will tout lots of pretentious spiel about "continental attitudes" and how monogamy is "unnatural".
These people should therefore not take social vows which SPECIFICALLY state that they will remain monogamous until the day they die.
Even if you didn't realise what strong views you had on the subject until after you were married, stuck in a rut, and someone who took your fancy came along (my, how convenient?), you still had a choice. You had a choice to sit down with your husband and say that you were struggling with this aspect of the relationship, and you needed more flexibility.
That would give him a choice. A choice to either engage in the same behaviour as you, or to WILLINGLY and KNOWINGLY accept that you did so, or to say that this was a deal breaker for him and to finish the relationship.
Also, I really wish people would stop putting forward "continental" views of marriage as some kind of elusive high brow justification of how society accepts adultery.
Spain and Italy are very Catholic countries, which subsequently have very strong notions of gender roles. The acceptance of adultery is a massive double standard, which ONLY APPLIES TO MEN. As a generalisation, many men have a huge madonna/whore complex, so when their wife has children, they become a mother and not a lover. A woman is expected to not ask questions so as to avoid the stigma of divorce, or risk her family unit. To present this as some kind of futuristic liberal ideal of marriage is frankly terrifying.
France too, isn't the free swinging free for all that people put it forwards as. It's true that adultery is much more acceptable here, but that is for OTHER PEOPLE. So, for example, if you found out that a colleague was having an affair, you wouldn't give a shit because it doesn't affect you. You wouldn't treat them any differently, or judge them for it, because it doesn't impact your life in any way. Likewise, if you found out a friend was having an affair, you would expect them to divorce, but you probably wouldn't cut ties with the adulterer. If YOUR spouse was having an affair, then - just like the UK - most people would either work through it or separate. It's not some magical wonderland where everyone is free to screw whoever and no-one cares. It's not permanently woodstock.
The only reason why Francois Hollande got more popular after he had an affair was because he is so fucking BLAND and DULL that people were amazed he was actually capable of it.