This is one of my guilty pleasure tropes because when the MMC does this he’s clueless at best and cruel at worst but I enjoy it.
The MCs are falling in love but for whatever reason the MMC doesn’t consider the FMC as wife material. Either she’s too poor, low-born, has a scandalous reputation or something along those lines. So he thinks it’s totally acceptable to ask her to be his mistress instead of his wife and it hurts the FMC’s feelings. He might even be considering marrying some other woman while keeping the FMC as his mistress. FMC feels disrespected and of course declines the offer and usually it leads to their relationship ending or being strained for a while until he eventually pulls his head out of his butt and realizes he should make her his wife.
I just recently read {How the Marquess Was Won by Julie Anne Long} MMc is a marquess who needs to marry a specific girl who has a piece of land attached to her dowry and he wants that land as it was once a part of his mother’s dowry that his feckless father gambled away. Also he needs an “acceptable” marchioness. But then he meets FMC, a penniless school teacher born in the slums and starts to fall in love with her. In a moment of passion he proposes she becomes his mistress and she’s super upset. She angrily declines and it obviously puts their relationship on the rocks for a while.
This also happens in {After Dark With the Duke by Julie Anne Long} where MMC is a duke and a widower considering marrying again. FMC is an opera singer with a scandalous reputation though the scandal wasn’t her fault. When they first meet he pretty much disapproves of her entire existence lol. But then they start a friendship and eventually sleep together and he proposes they make it a more formal situation where she officially becomes his mistress and again she is super hurt by it and declines.