"Trying to think of the responses this might get if my dad was a female OP"
They'd be different, I'm sure. But you are in a different situation - it's your parent, not your spouse, who's having this apparent affair, and I believe the advice has been quite well tailored to that.
I can only tell you, for the little it's worth, that XH was a devoted husband who idolised me and relied on me and as far as I know was always faithful. He was also negative, pathologically messy, emotionally abusive and often extremely silly. He drove me to the brink of insanity. After many years of holding everything more-or-less together I had a short-term sort-of online affair which believe me I do regret, but at least gave me the impetus to break out. I had screaming throwing fits while XH stood there looking sad. The family home which we had built ourselves had to be sold because I needed the equity to put towards my new house. (Don't I sound horrible!) XH gathered the DCs around him in a united front because they could not understand what I was doing, and the stories he told them didn't help (I tried for a long time to do the not badmouthing the other parent thing, but he had no such scruples; although sometimes he said I should be pitied rather than blamed as I was obviously having a midlife crisis). Most of the DCs were old enough to choose where to live so they chose to live with him; although they had begun to see I had my reasons, they reckoned he needed their support more than I did. Gradually, however, they all migrated over to mine because once I wasn't there to cover things up, or blame things on, they realised he was fucking impossible to live with. And they didn't even have to share a bed with him...
Oh, and the "affair" went nowhere and since I moved out I haven't felt the need to have one since. I explain to people, should they be so rude as to ask, that I've had aversion therapy to the whole bloke thing.
Probably completely different from what's happening between your parents, but all I'm saying (at considerable length) is: never judge a person till you've walked a mile etc. Even if you live with them you may never know the whole truth.