I'm not suggesting that someone in a functional happy marriage should "look elsewhere", someone in such a marriage wouldn't be interested in looking elsewhere.

I was thinking of the case where one spouse's needs were not being met. In that case the alternative to "looking elsewhere" is likely to either be simmering resentment on the part of the neglected spouse, or the other spouse feeling blackmailed into sex.

I'm really asking: If someoen is unable or unwilling to have sex with their partner, why shouldn't the partner look elsewhere? Who is hurt by that?

I know that many (most) people have very strong feelings about cheating - just wondering why.