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(+1)(-2)

That is the worst solution of all. It means that you will always miss out on one route, no matter how you choose. One of the main draws of harem games is that you're not forced to drop one route for another, that you can have all the girls and all the events if you want.

It's also the least flexible model of all. If you have events ABCD, then making A and C mutually exclusive allows only ABD and ACD paths, whereas optional routes allow AD, ABD, ACD, and ABCD. 

(-2)

Probably, i didn't explain it clear:
My thought: you making A event twice: A with vanilla kink and A' with BDSM kink. They are mutually exclusive, but has very same outcome. This way player will do A event but with the way he prefer.

(+1)(-1)

That's not the same event though, even if you use deceptive variable lettering. There's quite a difference between the same event and the same outcome. 

And that's only if I was willing to accept the false idea that the outcome can be the same when you're clearly continuing on a path with a different history of events - which I'm not.