Guys askin you out

Ok I have been in a relationship with my bf for almost three years. I love him and we’re serious. I just was wondering how do y’all ladies react when a guy gives u a compliment or asks to hang out? For example, some ex co worker guy messaged me happy birthday and that we should go out for drinks to celebrate. I either don’t reply or say sorry I have plans. I always feel weird/bitchy being like “I have a boyfriend” if they aren’t outwardly flirting. I don’t wanna be mean. But am I wrong for not doing that? Like what is the right thing to do?