The concept of sexual orientation is a social construct. Unquestionably. It's an idea that's pretty much unique to our culture (modern western countries primarily). Other cultures throughout history had different concepts that were quite different.
The fact that different people experience sexual or romantic attraction differently, and that not everyone is simply attracted only to the opposite sex, is not a social construct. It's a fact.
The concept of sexual orientation is a social construct. Unquestionably. It's an idea that's pretty much unique to our culture (modern western countries primarily). Other cultures throughout history had different concepts that were quite different.
The fact that different people experience sexual or romantic attraction differently, and that not everyone is simply attracted only to the opposite sex, is not a social construct. It's a fact.
So it can be both. Just like gender is both.