Welcome to our forum.
I don't necessarily forbid you to get involved with this couple, nor call it "abnormal" necessarily, but do be aware of the power dynamic and how much say you have in things. Sometimes when a married couple gets together with a single person, the couple makes all the rules (and they're not always fair).
Don't be in a hurry to make your decision; read and post some more on this site and learn as much as you can. There is a lot of collective wisdom here.
Glad to have you onboard.
Love means never having to say, "Put down that meat cleaver!"