This will go against all the other Mumsnet responses, because this topic always does.
I'm poly. I'm mid 40s. I have a long term female partner and a long term male partner. I also share a bed platonically, on occasion, individually, and often when travelling, with a good proportion of both my male and female friends. Some of them, we cuddle. Some of them, we don't.
It's weird to the majority because we're brought up to think that physical closeness and comfort is something one should only do with a single, monogamous sexual partner. But that comes from old Judeo-Christian morality, and you can choose for yourself whether that's something you want to care about.
If you can get past the social conditioning, then platonically sharing a bed and cuddling is a pleasant but not particularly important activity which carries about as much weight as a childhood sleepover. Neither of my partners gives a shit about it, because they're secure and well adjusted. And cuddles are nice.
And, when I have, in the past and on occasion, been in a monogamous relationship, I have spoken about cuddling others to my partner at the time, and actually, they've all been fine with it, because I don't tend to date people who don't challenge conventional thinking, nor lack trust or are insecure, so it's also possible to find people who are monogamous and are also fine with cuddling others, although I'll agree it's probably rarer than in poly circles.
Edited to add, some of the people I cuddle with are in monogamous relationships. And it only happens if their partners are also fine with it. It's not nearly as uncommon as the responses here will make you think.