The significance of the curtain closing seems to be exclusion. Rejection. It'd be thematically consistent with the preceding scenes.
Owen is first seen sitting on the jungle gym, alone, after dark, in the winter. This is apparently just before dinnertime, so one can presume it's not
late at night, but it's cold enough outside to warrant wearing a thick down-filled jacket and boots of a sort that can keep the feet warm in subzero temperatures (fahrenheit). He's not on the phone chatting nonsense with his buddies or sitting in front of the TV watching Star Trek reruns; instead, he's enjoying a barren view of darkened windows and an empty courtyard. This is apparently by preference, and it's possible this is his usual preference when he knows his mother is likely to be conscious.
He's seen next in the kitchen being questioned (but not grilled) about his day and whether or not he'd spoiled his dinner with candy. She's not really interested; he may be in the company of his own mother, but he's still alone. She forgets all about him when the phone rings, turns her back on him. Goes off into her fight, and then runs off to her bottle..
We'll not belabour the rather obvious sexual symbolism of the knife play in front of the mirror or the telescope. That's been done in
other posts. What's also been done in other posts is Owen's wearing a mask during the knife play, and it seems relevant here: Owen doesn't really want to be seen or known. If the condition a man's home is in is a faithful reflector of his inner weather, his is a chaotic, disordered emotional life.
This is the second time in the movie he's seen completely alone, and it'll be far from the last. When he's acting out what seems to be his most burning desire of the moment, he's not only alone but masked as well in the
one place most people feel safest: in his very own private bedroom.
His first peek through the telescope is at Virginia and her man who seem to be having some kind of dispute. He
lives in a dispute, and doesn't spend any time wondering what Virginia's deal might be.
His telescope swings over to the bodybuilder. This guy is no Adonis; longish hair, scraggly beard, what appears to be a soiled tank top tee shirt, the guy might be a truck driver, factory worker or maybe a lower tier dealer. Still, he's young enough, has big arms and is pumping iron easily enough. He moves with a kind of confidence and grace that suggests he's not worried about much of anything. Everything, one supposes, that Owen couldn't say about himself. Owen takes the mask off his face, puts it up on the top of his head, looks down at his body. He sees the same thing we do: small, skinny, powerless. Who could want a boy like Owen?
The telescope swings back to Virginia's apartment. They're not disputing anymore. They're calm; she sits on the couch, fiddles with some kind of crafts project. He joins her. They're talking. They're being together.
This is the good stuff. Her apartment appears to be more pleasantly appointed than his; more brightly lit, less cluttered. More inviting. The only physical evidence of disorder visible is the remains of a dinner or snack on the coffee table. The couple themselves are young enough,and clean. Virginia actually looks kinda classy in that robe-like thing. The man gets closer, starts nuzzling and kissing her, moving his hands over her body. She's responding, enjoying it. It's very slow and very tender, and it's clear this couple intend to spend some time savouring what they're sharing.
I sometimes wonder what I would have seen if I'd still been twelve and seeing this. It's hard to imagine that he'd have known less about human sexuality than I did at that age just twelve or thirteen years before the movie's setting, but it's certainly possible. Maybe Owen had no real idea
what might have been coming next (but he didn't have both hands on the telescope, did he...?), and maybe it wouldn't have mattered - at that age, I'd have worried about the sexual significance of what'd been about to happen
later. I think I'd have been immediately jealous that somebody is enjoying things denied me. Companionship and simple affection. Closeness.
This is the good stuff because this is what life is, the life that it seems Owen can only experience from a distance, through a telescope, a television or on a movie screen. Or, well, maybe also when a young couple unguardedly display physical affection at the arcade. Whatever, it looks like the only time Owen ever sees it is when people believe they're not being observed.
If Reeves had decided to have Virginia and her man rip off their clothes and bounce off the walls, the coffee table, flip the couch over and maybe break a couple of chairs before somebody thought to close the curtains, it might have lent itself to a different interpretation, but that's not what happened. A single breast was shown briefly in a moment of quietly languorous sensuality before Virginia opened her eyes and seemed to see right
through the night and into Owen's bedroom (which I seriously doubt).
Closing the curtain excluded Owen from whatever happens next. It barred his view into a much pleasanter world than the one he lives in. Rationally, Owen should have been able to tell himself that it wasn't personal, that whatever Virginia was about to get up to was a private matter, and that
nobody was invited to that particular show.
He was the one who'd been excluded, though; the curtains were closed on
him, just as so many other curtains and doors are closed against him. The expression on Virginia's face just before he ducked looked like one of disapproving judgement...
... and maybe a bit of an "eww" factor. What sane woman of Virginia's age and physical beauty is going to want to be lusted after by a bony little shrimp like Owen?
I don't believe there's any message here about Owen's probable future sex life, at least, not specifically or directly. It struck me more as being just one more device to help drive home the essence of Owen's isolation and feelings of rejection. The "sexual" component of the exclusion was superficial, just there to heighten the effect and to magnify the impact Abby's appearance would make on him just seconds later.