The game decribes itself a having quite a freedom of making decisions for the protagonist. So that is not true, because out of three romance options all thre are straight one, that's no freedom, that's boundaries... Not everyone is gay, right, but not everyone is straight as well.
You know as a person who has played this game I can tell your assumptions about it are not entirely correct. There are some gendered binaries in this game but that is heteronormative whether you end up single or not. Also, I play Fallout 4 and over there you have the choice to be bi, gay or pan and I respect that and love it! And yes, this is not a dating sim. I can say "awww" to some of the romance scenes but the bulk of the game is not at all dedicated to romance. And giving the setting and as I know the characters having a same sex relationship would to some people look disrespectful and it would not all give a positive message. Diversity comes in many forms like also having an ace or demi sort of orientation which is also not included in this game which is also something worth thinking about. And having a heterosexual interest doesn't mean heteronormative. Please understand that. If you date both sexes or all genders that still means you are LGBTQA+ and that means something. Also, if you as the protagonist date all sexes and end up with a het choice that still makes you queer. I really liked this game though there was some parts that seemed really gendered to me and I really wanted to thrash one of the characters it's still is pretty good. I actually wanted them to extend some storylines. Now, there are not 3 romanceable options. There are 2 and from the beginning it is your story as Daphne you can tell them to sod off. Personally, as I stated one of them made me wanna punch to tomorrow. And I am happy you can end up alone. You can say why not include the SGA dimension and to some extent that choice in this story may not have bode well. Trust me in the story it would look kinda "trying hard" but not building up on it. Also, it would be weak representation.
<SPOILER: SPOILER AVOID THIS AVOID THIS IF YOU DON'T WANNA BE SPOILED:
Some of the achievements depend on "WHO" you date. And yeah I had to date someone because I wanted that achievement though to be honest I wanted to punch his face to kingdom come. But get this some of our choices automatically makes it impossible to date someone even if you wanted. So, yeah, in the storyline the romance part is not important. This is Daphne's story and that is more important. Yes, if you commit to a relationship or some romantic interactions it does affect which baseline Daphne is emerging out of. That is a weakness in this game but not always. It just shows how you are treating people has some effect on who you are. Though I see some aspects of it related to criticism. Like Frank: that dude is a complete jerk!>
Also, technically, not all three are straight ones. There are two romance options, and the option to remain single.