While I'll never really understand the desire for one to have their children be related to them (does the opposite somehow lessen love?), there are still options for that depending on the genders/sexes and the preparation of the people involved, depending on the degree to which relation must be established.
For instance, a typical arrangement with a cis/trans male or cis/trans female pairing (i.e. gay or lesbian respectively) is that the trans person often is advised to have some sperm (trans woman) or eggs (trans man) frozen before they get on hormones, since HRT essentially will sterilize you. I dunno the specifics, but from there once you have the trans person's gametes, that's the main hurdle; from there, you could probably have a fertilized zygote implanted in the cis female or trans male partner, up to their willingness to do so, or look into surrogacy, and either way end up with a child that is 100% yours.
If it's a straight-presenting couple (cis man & trans woman, or trans man & cis woman), then you're somewhat shit out of luck here, though. You're stuck with having a child only related to one partner at most. (Frankly it seems like most trans people I've seen are somewhat resigned to never having their own children even if they have gametes on ice "just in case." So I imagine it's only the cis partner that's concerned here, and if so, and if they can bear with the child not being related to their trans partner, then there's a multitude of obvious solutions there. Obvious solutions also if the cis person doesn't care but the trans person can concede only a relation to themselves. Just need the appropriate donors/surrogates.)
(I feel like there have been advances in this stuff recently that may allow for someone's stem cells to be coerced into turning into gametes for the opposite gender of the donor as well, e.g. a man having a stem cell turned into an egg, or something of the sort. Though I imagine this is still very early-stage research and not considered a clinical option at this time, and that's assuming I'm not hallucinating this.)
In general, of course, adoption is also an option, but obviously no relation to the children.
That said: not wanting to date a trans person because that trans person in specific cannot give you biological children, and having biological children of your own is a life plan of yours is entirely valid. For some people, fine, that excludes basically all trans people. For some, it doesn't though, especially if you're not going to demand your children have to be 100% related to both you and your partner. And if it doesn't exclude all trans people, then it's problematic to exclude all trans people from your dating pool, because then it's clearly actually because they're trans.
The rationale I give this exception is because, well, infertility is not a problem unique to trans people. I can entirely foresee a scenario in which a cis person, who is infertile and doesn't have gametes on ice, gets rejected by people that want their own children. (Note: If you would date a cis infertile person, but would reject a trans person for not giving you biological children, then clearly the reason is actually just because they're trans and you might as well be upfront with that. General "you", btw, not necessarily the person I'm responding to.) At that point, it's a conflict of life goals and not really about who they are as a person, so it gets a pass.
Granted, I personally don't "get" why people insist on having biological children, because it feels like what really matters is raising them and giving them love regardless. I feel like a lot of people who parrot this same point probably would give hypothetical stepchildren of theirs crap or give them less just because they're not related. And there's also just tons of kids out there who need to be adopted regardless. So I question why people care about their DNA being present in a child they raise, knowing it came from their and their partner's genitals, when that's a detail you can easily just forget for their entire lives. I don't get it.
Yeah, I understand all of this rationale. I also think that many men use the biological children reasoning as an excuse to not seem transphobic. Not all, but definitely some of them.
Note that I am not giving definitive answers here, because I think this is not an easy topic.
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