Don't think you can yet. You get the chance to spend some time with them near the end of Lin's story. But you don't unlock 'have some fun' with them at all I'm pretty sure. Maybe next update!
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.