K.R.
I am assuming that her reasoning for not liking him is unjustified & that he hasn't "wronged" you.
I think you need to stop justifying him to her. In doing so, you just look like you're making up excuses to her & she feels vindicated. Tell her that this is the person you love & choose to be with & if she loves you, she will accept him. Tell her that this subject is off limits for good, period. If you need to, take some time away from her until she can accept him This is not her business, nor is it something she can control.