DePauw University and Wabash College announced Monday that they are joining Freedom Indiana to oppose the proposed constitutional ban.
The bipartisan coalition includes top employers such as Eli Lilly and Cummins, along with the Indianapolis Chamber of Commerce and the Human Rights Campaign.
DePauw and Wabash issued a joint news release saying a constitutional ban would stifle efforts to attract talented faculty and staff to their campuses.
Indiana University also opposes the move. Purdue University isn't taking a position on the issue.
The amendment to the constitution faces a final vote in the 2014 legislative session. It would go to voters next November if it passes the Legislature.