astrobunny-- hey, it's your life; and as you put it, you're an adult. (>21 years old.)
This is an old-fashioned word, but there are couples who at least used to be known as "swingers," or participants in an "open marriage." They wanted to keep the marriage going, just not be monogamous about it. If you've known this couple for only 5 months, you may not yet be perfectly clear about what this couple's motives are.
But I doubt that they're in this ménage à trois for your personal benefit, no matter how kind they may be to you.
I have to wonder, never mind Mom's hope that you'll find a monogamous rich suitor, why you don't think you deserve someone else's undivided love?