Yeah, we polys call it NRE, but of course, monos experience it too. It's called infatuation. Oddly, I was just reading an article about the biology of kissing in a Martha Stewart Living magazine, of all places, and the hormones, or neural impulses, that are activated by kissing alone are quite impressive. Dopamine, oxytocin, serotonin. All of these hormones cause obsessive urges to be with the person you had a nice makeout session with, never mind full-on sex!
Dopamine activates the same part of the brain that cocaine does. It makes you want more of this "drug." It can cause sleeplessness and loss of appetite.
It "allows us to recognize interesting situations, remember pleasurable experiences, and seek new ones." It brings euphoria (and I bet it makes you say stupid things like, "I've never felt like this with anyone else before," to your new lover, forgetting you felt that way for your primary once!)
Oxytocin fosters feelings of attachment.
Serotonin can cause obsessive thoughts of your lover, similar to sufferers of OCD.
The stress hormone norepinephrine is also released. It can cause the weak in the knees feeling.
Adrenaline is produced during deep kissing too, causing boosted heart rate, sweating, reducing stress, and "priming our bodies for more physical contact."
So! No wonder people in NRE or infatuation can forgot about all other people in the lives. For monos, it's common for people, especially teenagers who are new to sex, to stop seeing their circle of platonic friends for months, just wanting to be with the lover all the time, causing hurt feelings. For us polys, the situation is more serious, because it's the beloved primary and children who can get left behind in the dust.
I guess sex without kissing reduces the amount of these hormones in your bloodstream and brain though, which is why swingers so often don't kiss swinging partners, reducing the obssesive and bonding feelings, and reserve kissing for their SO.