Your post seems to be making a lot of assumptions.

You do not describe what you mean by cheating? How does one commit adultery online? What do you mean by relationships?

Why is it not acceptable to have a relationship with someone outside of marriage. If I had BDSM friends before I was married is it not alright to still have them afterwards? Is cheating just the act of knowing someone without the partner knowing them too?

While you did not say it directly, I got the feeling that you also assume that a BDSM relationship must include sexual intercourse. Many people have need in their life for a D/s relationship that is not dependant on sexual intercourse.

I personally do not think that causing any sexual stimulation during a session is commiting adultery or cheating. Especially when that partner then returns home with probably a greater sexual desire and certainly a greater sense of well being then when they left.

I would think a husband/wife would be happy to have his/her wife/husband racing home to jump his/her bones after a session with a good master/mistress.

Lets see what others have to say...