The ad from Stonewall was factually correct, and with homophobic attacks on gays, both physical and verbal, the statement 'get over it' has value.
The response from the Christian group could be challenged on the grounds that psychological, medical and psychiatric establishments all agree that sexual orientation cannot be changed, and that so-called "reparative therapy" aimed at altering gay peoples' orientations does not work and may, in fact, be harmful.
Should TfL be used as a platform to encourage these practices?
Who is the Christian group talking to? Do gay people have an issue with people being straight? Most probably not...I say that because there are so few stories of gays attacking straight people for their sexual orientation.
I can't recall every reading about gay people rejecting family members for being straight.
There aren't gay churches which condemn straight people for their 'lifestyle choices'.
So really, it's not gay people who have anything to 'get over'.