There is no absolute right or wrong answer to these questions. Such a decision is very personal.
Above all else, remember it is your right to choose if or when to discuss your gender identity, your genitals, or any other part of your body. If someone insults or attacks you because of their expectations about your body, that is NEVER YOUR FAULT.
NO. Plenty of non-trans people do not have discussions about their gender or their bodies prior to having sex. However, while you do not owe it to anybody to talk about your genitals prior to a sexual encounter, it may be safer to do so. It may also be less awkward or uncomfortable in general. So it may be ideal to have a conversation about the type of sex you would like to have beforehand. If this is simply not your style or not an option, that is okay. In terms of safety, there are advantages and disadvantages to disclosing in various situations. Your decision may be different depending on the person you are talking with and how sensitive or aware they seem.
The advantage of this is that you have a better shot at weeding out people who are unfamiliar with trans people which may mean not having to endure as many annoying questions on a date. Another distinct advantage is attracting people who are specifically attracted to your gender identity and/or your body type. Disadvantages? You may also attract people who eroticize trans people in a non-respectful way. If you feel any discomfort from the language someone uses with you in an e-mail, feel free to tell them so and/or ignore or block them.
If you are responding to another person’s ad, disclosing at some point during the chat or e-mail exchange is an option. Again, you have the advantage of getting the conversation at least started before you meet in person and if there is a negative reaction, you don’t have to deal with it face-to-face. A possible disadvantage could be that if someone responds negatively, they could potentially forward your e-mail or publicly ‘out’ you online or otherwise invade your privacy. This is why it is good to create an anonymous e-mail account.
Privacy violations or public outing are probably less likely to happen this way if someone responds negatively. Also, being able to hear someone’s tone of voice and gage their language use in real time can be helpful. E-mail and chatting can easily be ‘rehearsed’ and can be less authentic. A phone conversation can give you a more realistic impression of how sensitive or knowledgeable someone is about trans people.Some trans people, when meeting potential dates or sex partners in person, choose to reveal information about trans identity and/or body before a sexual encounter. Unlike an online conversation, you will be able to read body language and other nonverbal cues. Often times, discussions about trans identity can be accompanied by discussions of what the two of you do and do not want to happen, and discussions about safer sex.
A date can be an ideal time to talk about gender identity, since general discussions about your lives, background, family, etc will likely be happening. If someone responds in a way that makes you uncomfortable, you will be in public. Again, it can be brought up in the context of a safer sex discussion, or in the context of your past.
Some people choose to have gender identity, genitalia-related, and/or safer sex discussions at this point. It may feel like having the discussion prior would be assuming too much or that the opportunity just wasn’t there. Waiting until this point does in fact allow you to be certain that a sexual encounter is desired. One potential disadvantage however is that you may no longer be in public at this point if the person responds in a threatening manner. Also, sexual conversations can actually sometimes be less awkward when they are brought up in a non-sexual setting, like a public place. Sometimes it can actually be harder to talk about personal, sexual topics after sexual activity has already begun.