Jack has a girlfriend

We know that Jack is part of the LGBTQ+ community because she references her girlfriend a number of times. Whether she defines herself as a lesbian, gay, bi-sexual, pansexual, etc., is up for open interpretation.

Jack Flirts with Maura

Jack calls Maura pretty twice and both times it catches Maura off guard.

After Jack tells Maura she has pretty eyes...

She starts to ask Maura something and then asks something else. Do you think she was going to ask Maura if she is attracted to women? It's up for interpretation. Jack covers her face at the end of the scene which could indicate embarrassment or some sort of internal struggle. Perhaps this is because she wanted to ask Maura if she was attracted to women, perhaps it was because of something else, but it's all worth considering.

Jack tells Maura to go for it...

Maura tells Jack she likes her even though Jack has a girlfriend. Jack doesn't seem too bothered by the rigid structure of monogamy and gives Maura the green light anyway. One could consider Jack's opinion on polyamory vs monogamy using this excerpt below.

There is a universe in which Jack could by considered polyamorous too... check out that page here.