I'm not exactly defending him, but I understand his position a lot better than these 20 somethings.
Over a lifetime, if you're lucky and work at it, your relationship with your parents changes.
They go from dominating power objects to be rebelled against, to some of your closest friends.
I'm betting when her husband died, his mother changed, a lot.
Became more worried, fearfull, less independent, more needy.
This comes from being alone after 40 years, not a physical imparement.
Her son is trying to help her remain independent and out of a retirement home,
and is to be commended for his selflessness and caring, commitment, and basic family values.
At 78 his mother has had her beliefs about "sleeping around" for a long time now and isn't about to change them.
She's probably very comfortable being "the only woman" in her son's life,
and you probably represent a threat to her security.
Yes the age difference is a problem, but you sound comfortable with it.
He sounds like a good man temporarily struggling to make all the women in his life happy.
What part of what I just said makes you want to throw him away?