I'm afraid to say, darling, it won't have been his first visit, nor his last, for sure. It was his first 'mistake'. There's no point in believing he wanted you to 'find out'.
Take my advice (since you quite rightly wish to work on your marriage) read/listen to an expert (and ex sufferer) of the same in her early marriage - Joy Rosendale, who is now a psychosexual counsellor (as a result of her husband doing the same continually and eventually destroying their marriage). She was a guest on Ladies Hour with Jenni Murray. It should enlighten you.
Sadly in the professional workplace it is acceptable for men to openly discuss it, and crude comments relating to their sex lives, 'referring to a sex worker as 'a brass' - even in the presence of their female colleagues; it's a form of bragging to each other. The female colleagues accept this as i't's not their concern', and fear that complaining to HR dept may result in punishment from their male colleagues.
Otherwise, his behaviour will continue, albeit much much more carefully hidden. A telling off/plea for forgiveness will not suffice. This behaviour usually begins with viewing computer images, (iPads etc can easily delete history) then leads to 'home visits' as telephone numbers are advertised on every page (they even have forums where the 'clients' recommend certain workers!) then, eventually more 'unusual' activities begin because the female/male (don't rule it out) sax workers become 'boring' leading to eg LBoys/more sordid proclivities.. I know, it is horrendous when one learns this of their partner, but, despite his denials/disgust at even the slightest suggestion he may be interested in the latter, do not believe him. It will happen eventually if it hasn't already.
FYI It happened to me, but I eventually divorced, because it was too late. I wish I had been advised then as I am advising you now. Good luck