Detailed description of the rebirth process appeared in the Garbhāvakrānti Sutra and the Abhidharmakośa, with the former being written first. AFAIK, both of these texts are significant in Tibetan traditions.
"Lustfully drawn" is sexual attraction, judging from the relevant passage in the Abhidharmakośa provided by José Cabezón:
The eyes [of the antarabhava being], born from the power of its karma,
behold its place of rebirth even if it is far away. There it sees its
father and mother having sex. If it is [destined to be reborn] a male,
there arises a male's desire for the mother; [and vice versa].
Conversely, there arises hatred [for the parent of the same sex]. As
the Prajñapti states: "Either of two minds can manifest in the
[intermediate-state] at that time: it may possess an attached mind or
a mind filled with hatred." Confused by these two [thoughts], but with
a desire for sexual pleasure (rantukama), it clings to the site [where
the parent's two organs are joined] and thinks to experience sexual
The existence of an "intermediate state" was a contended issue among early Buddhist thoughts, as it countered the belief that rebirth follows immediately after death. Mahayana schools generally accept it. I'm unsure of how Theravadins perceive the issue, for it seems to vary by countries. Regarding this particular piece of text though, there are cultural (and perhaps, political?) reasons that might make Buddhists in East and Southeast Asia not want to discuss it (at least not publicly). It can be off-putting to hear that fetuses pretend to sex with their parents.
The next passage from the same source mentions a neuter (napumsaka) child. Napumsaka and neuter are not exactly equivalent, as pointed out by many Buddhist scholars, but I digress. The text goes on to describe the position of a neuter child. The implication I get is that a neuter child is initially conceived as either male or female but becomes neuter during gestation. There is no further explanation as for how or why such transformation occurs.
I'm unable to find further sources commenting on sexual orientation. The description above by Vasubandhu suits a genderqueer or intersex person more than a non-heterosexual one. It's hard-coded so that heterosexual male fetuses face their mothers as if copulating with them, while female ones face the opposite direction. I find it difficult to imagine which direction a fetus would be facing if it's bi or ace.
The earlier text, the Garbhāvakrānti Sutra, shares a similarly hard-coded system of heterosexual attraction/aversion. I don't have direct access to the sutra, but the articles I've read involving this sutra didn't mention the neuter gender at all. Maybe other users can clarify.