That's very true, but I do think that the key point is the dishonesty, and that remains the same. If BIL's being honest about his motivations, I agree that it's a different ethical calculus – but he was still wrong to do it without telling his wife (absent suggestions of abuse, etc., which we don't see here); in part because he exposed her to greater risk of STIs without her knowledge or agreement, and in part just because when you're in a sexually monogamous relationship with someone, you have committed to not have sex with other people in secret.