Astoria was, honestly, thrilled that her swimsuit had gone over so well. She loved the idea of keeping things fresh. Wizards had a long lifespan, so who wanted it to get dull? They had a lot of things to explore and she intended to keep it that way with their lives, if she had anything to say about it.
"I'm with a bloke who makes me feel happy, safe, sane - I feel alive. It makes me feel sexy." Astoria didn't have low self-esteem by any means. She wasn't validated by who she was with. All the same, she hadn't really ventured past snogging because no bloke had been worth the fuss. Draco was because he complimented her so well and helped not only to round her out, but he made her happy on completely unrelated matters. He was her friend first.