And if it was me, anyone "doing it" on my parents glass table, in front of my 2 minor sisters would have gotten it handed to them.
I guarantee his sisters know what sex is and didn't really care. And his 16 year old sister probably isn't a virgin. Wouldn't even surprise me if his 14 year old sister wasn't a virgin. They are not that innocent. But still I see you point, I'm just saying he didn't expose them to anything they didn't already know about or have possibly seen.
You seem to be an outstanding example of the human being species and will most likely be a valuable member to MFK