Is it normal thou shall not lust after what?

I don't get it either. It's so they can have control over your thoughts and feelings.

Shouldn't, cause it icky & grody & you might get cooties.

It goes along with "all things in temperance". When you're in a romantic relationship with someone, maybe they're into a sexual act which you're not into or vice versa, and in cases like this lust is kept in check because you'd rather have the person know they're safe and valued by you, to the point that you're demonstrating your willingness to forgo an act for the sake of your partner's comfort.

Lust becomes something bad you have a "use and cruise" mentality where all you want is a quick nut and you're fine with manipulating, hurting, or using another person to get some cheap sexual satisfaction

It can lead to cheating on your wife, raping, molesting, being gay or whatever. Spreading STDs, creating bastard children etc etc.