@Whoknowsohyoudo · Today 12:17
Exactly this. My DH is an ass grabber and it drove me crazy to the point I started bending at the knees instead of at the waist when I had to bend over. After months of asking him to stop I finally lost it and slapped the beard off him. He is much more restrained now. I don't think he meant you any harm and would be upset to know he did. If he's not completely contrite after seeing the mark, well that's a whole new thread.
Ditto. My DH used to do this. Thought it was cute and affectionate - and that I would be turned on by his whacking my arse, grabbing me and pushing me against the wall, and pinning me back whilst holding my wrists tightly. Also loved thrusting his pelvis against my bum when I was washing up, or cooking - and started writhing against me, and squeezing my breasts and pinching my nipples.
He was baffled when (after about ten times,) I shoved him away, and screamed 'will you fucking STOP grabbing my arse, man-handling my tits, and pinching my fucking nipples?! I am sick of it!' He pulled a face and pouted and said 'well if I can't touch my own wife, what's the point to anything?'>>> sad face 😢
I said 'I am not your property, you don't get to poke and prod me like play-doh. I do have feelings. And FYI - I HATE it.' He had this idea that he was somehow entitled to manhandle me and grope and prod me because I am his WIFE... And (as I said,) hilariously, he thought it would turn me on. 
Quite the opposite, I found it revolting and repugnant, and my fanny dried up, and closed up a little bit more, each time he did it. He also had a period of time, where he would slap me on the arse every time I walked past him. It got so bad that I walked past him 'frontways' all the time...
Or when I was walking away from him I covered my arse with my hands so he would just end up slapping my hands and not my arse.
He said 'anyone would think you don't want me touching your arse!' I said 'I don't. I HATE it when you slap my arse. It's as irritating as fuck, and I dread walking past you because I know you're going to whack my arse. It's not a turn on, it's a turn off. Just STOP IT!'
This was 25+ years ago, and thank fook he stopped after a year or two of doing it! But oh my days, it was annoying!
@Linguines As many posters have said, let him know how much you hate it and TELL him to stop. Also, as has been said, photograph the mark, and keep it somewhere safe.