People seem to forget how much the asari union means to Liara, and how special it is to her. The way she describes it as being such a deep, spiritual, emotional union between two trusting, loving partners who are completely content, comfortable and in love with each other. Liara is reluctant to be physically intimate with Shepard at all, she has to be absolutely comfortable with herself, Shepard, their trust and love for each other, and her own happiness. She even tells Shepard that she doesn't want to meld until after Saren is dealt with, but the night before Ilos she is so scared that she will never see Shepard again that she wants to meld then. "I have never been more sure of anything in my life".
It's testament to how much she loves Shepard that she's even physically intimate at all, as Liara is so shy and reserved, and unsure about melding. It's beautiful, and it's why the way in which Liara grabs Shepard's head to start kissing is one of my favourite Liara moments.