I do think it is entirely plausible he arranged to meet someone and they didn’t show so the condom was unused. Rather than he was going to the bank super early with the dog and a condom.
This^
OP - do you genuinely believe him, or are you just kidding yourself that you believe him?
Any genuinely straight man would never download grindr (sp?).
It will all rear its ugly head again. It can only be a matter of time. Ignoring it won't make it all go away, it'll just gloss over it temporarily. You can't make a bisexual or a gay man straight.
Say it happens again in 2/3/4 years...your children will be older, you will have wasted invested even more years in the marriage, it will be even harder.
I'm not one for hollering LTB, far from it, & I've put up with some shit it my time, but for me there would be no muddling through.
I'd have to cut my losses sooner rather than later, I'm afraid.