my parents think I'm dating my best friend??

I've been dating my boyfriend for nearly a year, but my parents don't know about him because they are very overprotective and nosy when it comes to my life, especially the romantic aspects of it. Last time they found out about one of my relationships they forced me to invite her over for dinner where they insisted we were confused about our feelings and we were just friends (we were both girls). 

My boyfriend gave me a box of gifts and stuff for valentine's day, including a card. I hung it up inside of my desk where they wouldn't be able to see it unless they were intentionally looking through stuff there. One day I came home and my mum had cleaned my room (this was an excuse to search it; she had made it even more messy) and I noticed the card was missing. Later that night I heard her talking about it and presumably showing my stepdad the card, they both tried to read it, but they couldn't make out a lot. I heard them go over all of my friends to try and figure out which one I would be dating. 

A few months later, my mum and I were having a disagreement and she said "I bet you just want to move in with (best friends name) because you two are dating anyway" and after that she would continue to try and pressure me into telling her who I was dating, I haven't yet and she doesn't even know my boyfriend exists. I just think it's so funny that they think I'm dating my best friend, they are so adamant that I'm a lesbian despite being so homophobic.