Crusader Kings III

That's certainly a quick way to do it, but there really is no need to set up a new game with unrealistically changed rules for sexual orientation just to get this achievement (not judging if you just happen to like playing with everyone being gay anyway :-) ). You can do this achievement easily on the side while playing practically any Iberian campaign.

While the name of the achievement might allude to a taboo secret sexual relationship, it doesn't have to be a gay one, you could even befriend and seduce your spouse for the achievement if you wanted to avoid getting an adulterer/fornicator secret.

You just need a culture that has the "ritualized friendship" tradition. All original Iberian cultures have that (Andalusian, Aragonese, Asturleonese, Castilian, Catalan, Galician). Having this cultural tradition, you automatically have access to the befriend scheme (without having to unlock the usual perk from the family diplomacy skill tree) and you can ask any of your friends to become your best friend. If they don't already have another living best friend and their attitude is at + 100, they are usually guaranteed to accept. First check whether they would be open to a seduction attempt by selecting the seduce option on them without actually starting the scheme, then become best friends, then do the seduction or romance scheme and you're done.