I don't believe that "heterosexual people" have ever been designated a "protected class" by the Government (groups are NOT automatically considered as such - they must have been designated as such by the Government, usually at the Federal or sometimes at the State level). However, "sexual orientation" IS a protected class under the Civil Rights Act of 1964, which was I believe the first major piece of Federal legislation about protected classes.

http://www.attorneys.com/discrimination/what-are-protected-classes

Despite what is often claimed by some people, many (I'd say most, actually) conservatives fully expected and also now do fully support the SCOTUS ruling that people of all sexual orientations should be allowed to marry, since that approach seems consistent with the equal protection clause of the 14th Amendment. 

"Obergefell v. Hodges...the fundamental right to marry"
https://en.wikipedia.org/wiki/Obergefell_v._Hodges