It made her feel awkward.
She was sure it was terribly politically incorrect and not-nice of her to feel this way and to blush every time she saw the two of them together (in the common room, curled up together in an armchair; at breakfast, their hands touching; in the library, their feet intertwined underneath the table) and to wish that they didn't have to be so obvious about the fact that they were together.
They hadn't made an announcement or publicly proclaimed their love for one another, but anyone who had been paying attention had noticed that there was something going on. She hadn't thought people were serious when they raised their eyebrows and said things like, "They seem to be very close, don't they?"
Lavender had brought up the topic when the two of them were alone one night. "Parvati? What do you think is going on between Hermione and Ginny?"
And she had replied, honestly, "I don't know", and hoped Lavender didn't press the subject, because even though there was a part of her that did want to know what was going on and liked talking about these things with her friend who, it seemed, was being terribly open-minded about the whole thing, there was another part of her that felt awkward.
When she overheard Harry and Ron talking about it, quite casually, she realised that they must be together, but of course everyone else already seemed to know, or at least have taken for granted that the intimacy between the two was of a romantic nature.
When she knew that definitely, without a doubt, Hermione and Ginny were going out, it made her feel awkward. She didn't know what to say to Hermione, if anything, and she certainly didn't like being around the two of them when they were being a couple.
But the awkwardness she could handle.
It was the jealousy that was the problem.