Julia offered up a possibility and this is part of her post is something I wanted to comment on:
For all we know, the marriage could have been horrible and the real reason that Sam got into the accelerator chamber (in the changed history where he is married to Donna) isn't to prove his theories or risk losing funding but maybe the real, never revealed reason is that he was running away from a looney tune wife and a bad marriage.
As much as this may be a possibility, Julia, I have to argue against it. My problem with this possibility is that Sam later reunited Donna with her father. IIRC, he knew of Donna when he encounters her as a student and he has a desire to reunite her with her father and believes this to be the cause of her insecurity which led to her abandoning Sam at the altar. I find it just a bit odd that he would want to get in the accelerator to escape from Donna because if he was originally married to Donna, he would actually want to
prevent Donna from being reunited with her father. Perhaps you might counter on this point that if he might have thought that by reuniting Donna with her father, this would help to cure some security deficit in her and help to improve her personality so that if he found himself back with her, ever, then she might be a different and more pleasant person to be with.
If this was raised as a counter-argument, I have a counter of my own and that would be that he seems to have a memory of being stood up at the altar and not that he had a crappy marriage that he was desperate to escape from for sanity's sake. I keep thinking that he would've remembered that he had a lousy marriage. Reuniting Donna with her father would have been for her sake if it was a lousy marriage and not for his sake, which was to prevent the heartbreak of being left at the altar and making her go through the marriage.
Maybe his desire to reunite her with her father was the desire to repair another tragedy in his life in addition to losing his father, Tom, and seeing Katie marry Chuck. I seem to like this alternative better.
Matthew