We know that she was sexually attracted to men in that universe. We don't know whether or not she was straight, although AFAIK she wasn't shown to be attracted to women.
AFAIK there is a Walkyverse character (I think Danny?) who was never shown to be attracted to the same sex in the Walkyverse, but is bi in DoA. Basically, while Willis can't make a presumed straight character from the Walkyverse gay while following that rule, he can make a presumed straight character bi and it still work, especially if they're attracted to a character that they never met in the Walkyverse (and Mary never met Ultra Car in the Walkyverse, AFAIK).
Edit: Added "presumed" in places.