Yes, I think that if you spend most of your life single in a world where romantic relationships are exalted to the extreme (because romance sells movie tickets and feeds consumption), you are likely to question your value in the sense of physical/personal attractiveness.
The same is true if you do have relationships and get cheated on, abused, dumped, etc.
But for sure, being in a relationship - knowing that you could attract SOMEONE - is validating in a certain sense. Just that it's not quite as straightforward as it seems.
But then you have the question of fulfilment. Now we're talking about something different. Because fulfilment absolutely does NOT come from a relationship. Fulfilment 100% comes from within. It's 100% about pursuing paths and experiences that bring you joy - whatever they might be. I actually experience more joy now that I'm single than I ever achieved in a relationship (and I'd been in a lot of relationships, previously).
I was in relationships, from 16 to 33. In some ways, I felt validated. In other ways, I felt rejected. And I'd say my self-esteem was LOWER than it is now that I'm single.
I've now been single for nearly four years. I feel more confident than ever, more at peace, more excited about the endless possibilities / blank canvas kind of vibe ahead of me. But I'd absolutely be lying if I didn't occasionally question whether I'm attractive to others. This doesn't spill over into my overall self-esteem, though. It's purely about whether I'm sexually appealing - wondering what people think when they see my face, speak to me, etc.
Physical attractiveness is just that. Personal magnetism is just that. Self-esteem and self-worth are deeper constructs. They aren't resolved by being in a relationship.