Regina is not wrong here, May is.
She lead her a bit too much and made a bit too many shocking revelations here.
First she takes Regina in the bath, then she complements her, then reveal the secret that she likes her and is attracted to her sexually, she also took advantage of her and kissed in the past, and she would gladly eat her out cause she would do that to all interesting people/friends loving them at the same time.
With such appraoch she basically pushed Regina expactations unknowingly to the limits, and in the end she shoots her down saying that she have to stay faithful(I kinda find it strange that she didn't say I want instead of I have) to Bern cause she loves her the most and she forces herself for their relationship to work.
And then as a result it is not surprising at all for me to see how Regina acted.
May acted a bit selfish here on that telling the truth high status, not considering even for a moment that Regina may like her too like that.