It is firmly established throughout Deep Space Nine that joined Trill inherit the memories, skills, and personality traits of all Trill who had previously been joined to their symbiont1.
We observe that for Odan and Dax, feelings of romantic and/or sexual attraction are among those traits that are inherited from one host to the next. We infer, but do not observe, that gender-correlated traits are also inherited, as Jadzia seems to gain some of her more masculine proclivities specifically from Curzon.
Logically then, the gender and romantic/sexual orientation of a joined Trill is, like every other aspect of their consciousness, a synthesis of the genders and orientations of all of the symbiont's past and present hosts.
The Dax symbiont's first host was Lela Dax (she/her). When Lela was joined to Dax, it's unlikely that her sexuality or gender changed in any way as a result of the joining, because the symbiont had no other experiences of humanoid sexuality or gender to contribute to her psyche2. When Lela died and Dax was joined to Tobin (he/him), he inherited her memories, including those of her lived gender and orientation. We don't know how, if at all, he incorporated those memories into his own life, but we do know that Lela's feelings persisted, and continue to persist as aspects of Jadzia and Ezri, as demonstrated in the episode "Facets" where we see Lela's identity reified and transferred from Jadzia's body into Kira's.
Let's fast forward to Jadzia. We can observe that she is bisexual3 through her relationships with Lenara Kahn (in "Rejoined") and Worf ("Looking for Par'Mach in all the Wrong Places", etc). It's possible that she was already bisexual before being joined to Dax, or she may have been exclusively homo- or heterosexual or asexual before being joined, and became bisexual as a result of incorporating the sexualities of her symbiont's previous hosts. We know that Curzon was (very) attracted to women, and that Emony was attracted to men, as evidenced by her having a relationship with Dr. Leonard McCoy ("Trials and Tribble-ations"), so either explanation is possible.
What about her gender? Jadzia is assumed to be a woman throughout the series, although Sisko does jokingly refer to her as "Old Man". I can't think of any examples where she declares a gender for herself other than "woman." Perhaps she is genderfluid, or genderqueer, or polygender--or maybe she's just a woman who remembers what it's like to be a man, and incorporates some aspects of that experience into her life without deciding that it warrants adopting a new label for her gender.
The fact that the Dax symbiont has lived as both a man and a woman could be interpreted as observing that the symbiont is genderfluid across the context of multiple lifetimes. However, this doesn't require that Jadzia Dax is genderfluid within the context of her single, joined lifetime. She could be, but it isn't logically required. It's possible for the symbiont to experience multiple genders across multiple joinings while each individual host retains a consistent experience of gender throughout the time that they are joined to the symbiont.
So, is Jadzia Dax genderfluid? I think it's open to interpretation, and ought to remain that way. Applying labels to fictional or historical characters who did not exist in a context4 that recognized those labels is a touchy subject, and personally I'm not comfortable making assertions about Jadzia's gender based on my own interpretations of how she expresses herself 5.
What about other Trill? Well, let's imagine an alternative universe in which Tobin was never joined to Dax, and every host from Lela through to Jadzia was a woman. In this timeline, Jadzia's experience of gender would still be a blend of the experiences of all of the previous hosts, but since the Dax symbiont would not have any memories of being a man, it's unlikely that being joined to Dax would result in any increase in masculine traits.
In conclusion, what we observe of Trill gender and orientation in Star Trek implies that most (not necessarily all) joined Trill are circumstantially (not inherently) queer as a result of being joined. Any symbiont who is consecutively joined to a homogeneous sequence of hosts who all share the same feelings of gender and attraction will not accumulate the diversity of such feelings exhibited by other Trill. This is vacuously true for all Trill who are the first to ever be joined to a symbiont.
1 I am assuming that any contributions from the symbionts themselves are negligible, since their experiences prior to their first joining would be largely irrelevant to humanoids.
2 Then again, perhaps every joined Trill is actively expressing worm gender, and we simply lack the frame of reference to recognize this.
3 Synonymous with pansexual.
4 That context being "American television produced during the 1990's", not necessarily "The United Federation of Planets during the 2370's".
5 If the Trill were a real species, I would expect them to have their own unique language for describing the gender of joined individuals, rather than use any of the terms used by humans.