Maybe they are not insecure?

Be as secure as you will, if you see your SO in a romance scene in a typical hig h profile movie with all the right camera angles and music and all... it's bound to do at least something to you, right?

No that’s the point. If you are not insecure and trust your partner then you know it’s just acting.

I'm not suggesting that they might assume any of the acted scene had real feelings behind it. Yet still, visually seeing your SO doing intimate stuff will make you feel something. You might not be jealous or anything, yet still. That has nothing to do with insecurity. You can feel awkward without jealousy, can't you?

As a former polyamoric person, here’s how I (still) think of it: You can’t control your feelings (they are merely physical reactions to situations) but you can control how you deal with them. Or how you act on them.

So in a sense, in my opinion, you’re both correct.