Nia has a fair amount of content in her Love Route, Sylvia is still lacking. You can click the "i" button at the top then go to "misc info" to see how many events you have left.
Sex will be attained slower in the love route.
The "let's have some fun" line is a placeholder for future sex scenes. Love route is just that - not focused on sex. Sylvia is clearly fairly stuck up, less so after the events of the island, so I imagine she'll be getting some content soon. Besides, Nia is a precious cinnamon roll and while I would tap the hell out of that I'm more interested in her story and helping her mission.
Ann was given sex scenes in v11, which is great because her dialogue previously indicated she was disappointed by the lack of it. Peni is understandable in jumping straight into it (especially because she's ultra fuckin hot) but personally I think the sex could've been delayed there too.