It’s super unclear, lol. He could be bisexual. But he’s definitely not straight.
Basically, from what my friend in Japan says, what Reiner says to Ymir is really unambiguous about him being gay, and Isayama has drawn art of him as well that very much suggests he views the character as gay (it’s explicit). Reiner’s birthday is also commonly known as “yaoi day” among fandoms in Japan, so, well, yeah. I doubt the character is intended to be straight.
It’s also interesting that this is Reiner clearly in Marlayan Reiner mode, when he tells Ymir he does not like women. When he claims to be interested in Christa, he seems to more or less be in his persona of ideal soldier then, and so it is possible he projects his ideal self as being straight, which is sad but not entirely unexpected given the toxic masculinity that often comes with the heroism ideals of Reiner’s ideal self. His ideal self is attracted to a mirage, because of course, Christa does not exist, and he still sees her as Christa as shown in how when he gives Ymir’s letter to the SC, he says it’s for Christa, not Historia.
Does/did he actually like Historia? I think so, because can you say that the role he was playing really wasn’t in any way him? I’m not sure it’s so clear-cut. In his persona, Reiner might be trying to simplify things into black and white, yes or no, when nothing in the world is and that includes his sexuality. He might be bi with a preference for men, but I do think he’s absolutely not straight according to canon.
As for him and Bertolt, well, we don’t really know about Reiner’s feelings for him–which, tbh, I do think are very ambiguous, and given his statements about his sexuality, it’s not wrong to interpret it as romantic affection on Reiner’s part, but of course it’s not canonical either. And his feelings may not have been reciprocated, though Bertolt clearly did care a ton about him, but that doesn’t mean Reiner’s feelings for Bertolt were totally straight. We just don’t know. But we do know/can say that Reiner isn’t straight.