I'd probably be a variable combination of irritated, hurt, confused and uncomfortable if I were your friend and you instantly stopped hugging me hello/goodbye etc. once you starting seeing someone, after an extended period of being happy to do so/initiating it pre-partner.
- Not because I'm entitled to physical affection/comfort from you
- Not because you aren't entitled to enact or change your physical boundaries
But because:
a. You've gone and made it WEIRD.
You were happily engaging in and initiating hugs pre-partner.
Absent a DV/controlling partner situation, immediately stopping once you start seeing someone then retroactively makes all of those other hugs you gave me 'weird' - it implies that every single time you were hugging me before getting a partner, you were actually getting something sexual/romantic/non-platonic/otherwise inappropriate to the parameters of our friendship out of it, and I just didn't know it. That would bother and confuse me and make me really uncomfortable.
And/Or,
b. I would feel used
It would make me doubt and second-guess exactly what I meant to you and just what our 'platonic friendship' actually was about for you up until now, because clearly you - consciously or otherwise - were inappropriately using me to get your physical and/or emotional affection/validation needs met whilst you were in-between relationships.
I mean, if your gestures of affection weren't (some degree of) inappropriate to our ostensibly 'platonic' friendship in the first place, then it wouldn't make any difference whether or not you were in a romantic/sexual relationship... The only way your being in a romantic/sexual relationship would both immediately eliminate your desire to 'hug' me and engender your perception that it would be 'inappropriate' to do so would be if our friendship dynamic wasn't truly platonic from your end.
If I was your friend, I'd be carefully re-evaluating our 'friendship'; I don't know that I'd be able to shake that 'I was just inexplicably dumped from a relationship I didn't think I was in' feeling.
That said, at this point the awkwardness has happened and you can't really unring that bell...
Plus, if you're uncomfortable, you're uncomfortable, and you're entitled to that, regardless of how bizarre I might find your reasoning. That's the beauty of boundaries - you don't have to like or understand them, you just have to respect them.
He's a grown man and he'll have to either respect the new parameters of your relationship, or move on.