What I mean is: How exactly do you define "enthusiastically participating"? It's hardly an objective criterion.
Tearing his clothes of? Crawling on top of him?

I do get what you mean...and yes, if she's just sitting there, being subjected to something, then yes that's not enthusiastic consent. But some people are fairly demure, even sexually. "Enthusiasm" cannot be measured objectively, because all people have different standards.