The writers' options were limited. Couldn't have written Mac as being gay or bisexual. Ergo, his orientation had to be straight or asexual. Back in the mid-80s, there was very little awareness that some people are naturally asexual.
So, heterosexual.
I'm assuming RDA was joking. I don't think there was any doubt about
's heterosexuality. Who remembers this particular bit from "Passages" where he is distracted by a woman? Imagine the director going over the directions with him: "So, Rick, Mac's actually dead at this point, and about to be reunited with his dead parents for the first time in many year, and he takes the time to stare at a woman in a tight dress. And that makes him think that the afterlife might not be too bad."
MacGyver: so heterosexual that he's thinking about sex even when he's actually dead.