Don't be daft, he couldn't explain it, even if he could get Mary to understand it and accept it, it would change his relationship with her for the worse. There is no need to explain it. It has been done in films and TV programmes over the years where someone has an ability or power or money and although they themselves can handle it as they feel the responsibility and yes, burden of the power...BUT their friends or partners seem to imagine the power to be endless and without the responsibility of holding that power they want unrealistic or dangerous things done just because they can.
For example in the UK Time travel comedy "Goodnight Sweetheart" Gary Sparrow can go back to the second world war, he has a friend who just wants to use and abuse the power and as soon as he has an opportunity he makes a total mess of the whole thing. Not saying Mary would be that bad but how could you ever look at any situation and not think "you've used your power" even if he hadn't. It would ruin Mary's life and that of their children. Nothing good would come of it. Mary would be forever looking at their children and wondering if they could do it.
Tim is, in my opinion, being quite strong about it. It would be tempting to tell someone, but I think he is mature and decent enough to get the responsibility of the whole thing. And also his dad didn't tell his mom and that worked out for the best, She loved him in ignorance, knowing would only have made it worse.
In a way it's like people who have an affair and can't wait to unburden themselves on their partners thinking they are doing the right thing "being Honest". It hardly ever works out. You think you'd want to know, but really you wouldn't.
Mary wouldn't for instance say "Oh that's nice dear, thank you for telling me" and get on with her life, she would have to reassess EVERYTHING.
I agree with your points but in this day and age when we expect the highest levels of honesty in a relationship would Mary be upset if she ever found out the truth about his ability to time travel?
I get that this isn't the kind of information that you casually pass out, I'm just wondering about the moral dilemma of it all.
Would you be comfortable knowing that your relationship with your significant other was built on the non-truth that it developed naturally/normally?
It's like getting a D on a test and then taking it multiple times until you got an A.
If I was in the relationship and got manipulated the way he did to her I wouldn't feel very good about it.
It would cheapen the relationship for me knowing it developed that way.
He didn't explain it because he had already used time-travel deceptively to win her over.
She could start thinking about the following and really get mad at him:
How did he know I loved Katy Perry?
How did he know my name was Mary?
How did he know exactly how I enjoy lovemaking the first time?
Lots of other strange coincidences about that 1st encounter.
No he didn't. He's a psychologically controlling monster, who uses his godlike powers to manipulate her life.
I don't get why people think this is a romance -- it's a horror film.