why does being a bisexual woman (22F) mean my boyfriends (24M) will always expect a threesome?

every single relationship i’ve had where i’ve been open about being bisexual, one of the first questions i’ve been asked is if i would or have had a threesome. i just want a sacred relationship and don’t want to be sexualized based on my sexuality. it just makes me feel gross. am i being dramatic by getting really upset about the question? i guess i just want to feel like im the only woman they think about and want to have sex with. i understand being curious, but i think it’s something really hurtful that bisexual women don’t talk about enough.