This is not an eroge. Sex will not be added in any capacity.
Additional buildings, objects and species may be added one day, however!
It might be quite some time until that day comes, as other projects that generate income take precedent...
As for not interacting with buildings: the small homes are open-front and they can easily be witnessed sleeping within them when the time comes, and they do indeed enter the castle-type building occasionally, although you cannot see inside that one.
If you ever happened to notice one of your Lintwians missing from the play field, they may be inside that building!