It’s because the romantic ideal/fiction is that lovers will both close their eyes while they kiss, as a result of (and signalling!) an intense focus on the tactile act of kissing, perhaps demonstrating trust and vulnerability at the same time. To kiss with eyes open implies detachment or insincerity on the part of the kisser; you’re going through the motions, but you’ve got your mind on something else and aren’t fully engaged. If your eyes are open, you might as well be checking your phone for messages while you’re kissing. It’s dismissive. That’s the common view, anyway.
As another singer, Pat McCurdy, put it:
“Why don’t you kiss me with an open mouth?
Why do you do it with a pursed one?”
If you absolutely must keep an eye on the treeline where the zombies might come through, at least have the decency to close the inside eye that your partner might see if they peek.
hm, mrAru and I kiss eyes open, and we screw with eyes open, we like seeing each others faces and expressions. No idea if that is good or bad but we have been together for about 30 years …