Yes this person I was dating just couldn't imagine anything else than casual sex or committed relationship. For me, there is a lot in between we could have enjoyed. But he didn't see it.
Most monogamists don't realize there is this thing called the relationship escalator. They just think the goal is to ride it to the top, in every prospective relationship. Basically, the escalator goes like this.
Meet (online) and start chatting
Move to phone calls, Facetime
Meet in person through friends, or at a bar, or at the grocery store, or hobby club, etc.
One of you asks the other to go on a date
Meet in person, first date
Start dating regularly
Or... Meet online and have a long-distance relationship where you only see each other X amount of days a year. This can go on for years or even forever.
If meeting regularly (and not asexual) start kissing, making out, fondling
(or have sex on the first date, of course)
Start having penetrative intercourse
Start to spend one night together, regularly, or not so often
If that goes well, start to spend a few days in a row together, say, a weekend now and then
Start to spend overnights together regularly
Declare to each other that you are official (bf and gf or whatever gender you are)
Fall in love
Start to say "I love you"
Tell your friends and family about your dating partner
Introduce your bf/gf to friends
Introduce your gf/bf to family
At this point on the escalator, this might be the floor you stay on forever. Dating, hanging out on weekends, doing things with your friends and family, like dinners, going to shows, sports activities, and so on. But you each maintain your own home, and you don't spend every day together. Or you may stay on this floor for years and then finally move up. If you decide to keep riding the escalator up, you proceed to:
Start celebrating holidays, birthdays, other important events, together with your families
Start taking vacations together
Move in together
Get a pet together
Perhaps get a joint bank account
Buy a house
Grow old together
Now, if you are poly and already have a partner or two, and start seeing someone new, you will have to take more people's wishes into account. Maybe you're poly and never want to live with any of your partners. You like having your own space. Or maybe you're in long-term relationship, or married, and already live with a partner. Will you also want to live with a newer partner eventually? Maybe you will go between 2 houses (half the week with Partner A and half the week with Partner B) or maybe both your partners will agree to live with you. If married, maybe you'll divorce your spouse (while remaining partners) so that no partner will have more legal rights than the other.
So, that's the relationship escalator in a nutshell; the grey area between casual sex and full-on commitment.