With the arguments on marriage equality due to hit the Supreme Court at the end of April, many groups are filing briefs regarding the subject. (IANAL and I don't even play one on TV, so forgive me if I screw up some of the terms.)
Among those filing amicus briefs in support of marriage equality are sports teams. One of those is the reigning world champion San Francisco Giants:
Some of the states in which amici do business make marriage equally available to all of our employees and colleagues; others prohibit marriages between couples of the same sex and refuse to recognize existing same-sex marriages. This dual regime burdens amici. It creates legal uncertainty and imposes unnecessary costs and administrative complexities on employers, and requires differential employer treatment of employees who are similarly situated save for the state where they reside.
You can read the full brief
(PDF) online.
Reportedly the Tampa Bay Rays (MLB) and New England Patriots (NFL) have also filed briefs in support of marriage equality. Would be interested to hear if other teams follow suit.
In the wake of comments by New York Mets player Daniel Murphy, where he said that he disapproved of the "homosexual lifestyle", this action by at least a few individual sports franchises is encouraging.