They are avoiding endorsing the other flip-flopping senators because they fear it will hurt their reelection chances in the Republican primary September 13th:

The Empire State Pride Agenda, the leading statewide LGBT lobbying group in New York, announced its endorsed candidates Tuesday morning, but only one of the three Republican state senators seeking re-election who voted for marriage equality appears on the roster.

The group backed Mark Grisanti of Buffalo, one of four Republican senators who voted for the bill that passed last year. Senators Roy McDonald and Stephen Saland have not been endorsed in their re-election bids. Senator James Alesi, the fourth yes vote, announced earlier this year that he would not seek re-election.

... All three Republican senators lost the support of the Conservative Party, which provides an important cross-endorsement in the state, because of their marriage equality votes, and they face primary election challenges on September 13. Grisanti’s primary opponent has not taken a stance on marriage equality, according to State of Politics, whereas McDonald and Saland face challengers who oppose it, which could make the endorsement from an LGBT group problematic. Democrats, meanwhile, are targeting Grisanti in the general election, where his marriage equality credentials could provide a boost. -- The Advocate