Bingo... and my thoughts exactly as the writer!!
The wife has been screwing around behind the MC's back for a little over a year, freezing him out of the bedroom and her life.
I think, deep down, she's unhappy still being married to the MC, but because of family pressure is remaining in the marriage... so that deep seated anger at being forced to remain married to a man she no longer loves (and back with the old flame in her life) is causing these little "blow-ups" in their marriage recently and why the MC feels like he has to walk around on eggshells around his wife.
Then, watching the "kids" and being sexually tormented (after a year of jerking off to get any release) and hit on by his niece, the MC cracks... and gives into temptation.
Is it wrong? Yes... even he is of the mind that he did something wrong. Could he be considered justified by doing it, yeah, I am sure that viewpoint could be argued... now add to the fact he finds out that he is the Antichrist and has to do "it" to remain powered up...
...I expect the MC to feel some guilt in doing this, believing that he is betraying a wife that he believes is being faithful to him... how he will react when he finds out (and he WILL find out... after all "your sins will find you") is going to be up to all of you to determine as the MC...
...both are in the wrong, make no mistake... but the morals of it and the outcome is up to you and your own personal morals.