Sometimes I use an 8" black dildo (I'm 6") on my wife when we're in 69 and tell her it's her bf's big cock. She really likes it, but this may be as far as we ever take it.
If I found out that my wife was cheating I'd let her continue until she was ready to talk to be about it. I'd love for it to happen but would want her to be totally comfortable with the situation.