Not that, What I am asking is the vaginal sex? I'm playing the sharing route. So maybe I have to be in the love path and avoid any ntr ?
I've only played that scene on the cuckold route. But my guess is, you wont get that scene if the MC is on the cuckold or sharing routes. On the love route, Lucia acts very differently towards the MC because he is more dominant. Because as she says, she likes bossing men around. So if the MC isn't totally dominant, she'll take that as her cue, to be the one calling the shots. Which in the case of the cuckold route, probably means no penetrative sex ever. Remember, Lucia is already cuckolding her husband Roy, and all he gets are handjobs.
On the sharing route, I think it will happen eventually, but if the MC acts as a cuckold then Lucia will continue to tease him, and deny penetrative sex.