For me, the litmus test isn't so much the age gap (Though having said that, sorry but a huge age gap does not seem "right" to me - however a big difference is that I will always keep my nose out, what works for me doesn't work for all, and it's that couples business not mine)

The litmus test, is more crucially "is there some sort of politics or control thing going on"?
Is it about love...........or is it her putting up with some bad tempered ugly old git because she wants his money?
Is it him going to Thailand and finding a "bargirl" ( ) to marry because he is a complete lowlife, too fond of giving backhanders if his Stella isn't chilled ok, who no self respecting UK girl would give the time of day to.

Nothing to do with love, nothing to do with respect and everything to do with just "using" the partner to get something they otherwise wouldn't ever manage........and this works both ways I guess.

I don't want to rain on anyone's parade, it's simply none of my business. There is though very good reason the old man and young tart gag was used repeatedly in Benny Hill shows