So they're being overzealous by calling them transphobic, but I can see where they're coming from.

Uhh, disagree. Many of our gendered issues with dating and relationships stem primarily from people having "expectations" (aka feeling entitled to certain things). One of the gender roles that causes this is that men should be flattered by any and all female attention, and any guy who has a girl "come on" to him is "lucky". Not only are they being overzealous, but they are actively working to impose entitlements into romantic life, which is counterproductive to the goal of removing harmful gender roles from dating and sex.

As to the use of SJW, a term that identifies people who take social justice to counterproductive extremes is not an issue. The problem is that the reactionary crowd likes to brand anyone who is ever offended or harmed by any part of the traditional cultural system as a SJW, even when the grievances are legitimate, in order to discredit them.

In this case, I'd say it is appropriate to label someone with the "SJW" label for automatically getting upset at anyone who turns down the romantic advances of a trans person.

Edit: Of course, this is all taking the OP in the thread at face value. Since we're only getting his side of the story, it may very well be the case that he really does harbor some transphobia, and that his transphobia at least surfaced when 'Rachel' revealed her feelings, and that her friends anger is justified.