I am sorry we live in a world where parents alienate their children the way your parents have. This is sad.
Tell your parents where you are at and the truth of your life before they come. They need to know and you need to say whom you are and what it means for them to visit. Be clear and straightforward. Explain you have a life that they are welcome to visit, but that there are some guidelines.
Explain that you love them, but that they are only welcome if they can respect your decision and can honor that decision by treating your partner well. Explain that such a visit is acceptable if they can do that, but that you are not willing to hide who you are to them. Tell them that you are happy and thriving, and that you want to share more of your life with them, but these are the ground rules.
How they respond is up to them, but you will have done all in your power to be open, honest, and clear with your needs.
“I love you, but until you are able to accept me the way I am, and treat my partner with respect and dignity, we won’t be seeing much of one another. I’m sorry about that, because I love and miss you, but I won’t go back into the closet to help you feel more comfortable. This is my life and I want to live it honestly and with someone who I love and feel better with.”