So my experience is from the perspective of someone who likes going down and who has had partners with pussies that were insecure about it. Patience is really key. The person with the insecurity needs to feel comfortable to start branching out and experimenting so focus on creating emotional safety for them. Let them know you love the thought, but will respect the boundaries they have set. Once that trust is there you can discuss options with them to lesson their anxiety.
If they are uncomfortable about the taste or smell, ask if a barrier or having the opportunity to shower before would help. Maybe a lubricant or other body safe fluid with flavor could be used to help ease them into it.
Set safe words for the experience so if they get overwhelmed or need to stop they don’t feel pressured to let you continue.
Discuss if they would prefer an experience where they guide you or if they would prefer you to take the lead
If the opportunity to shower before would ease their mind offer some shower fore-foreplay to get them excited
Be honest about anything that might bother you, but not judgemental. If you let them know what would actually turn you off they might feel more secure in where they are at. From personal experience, I am fine with most all vaginas, but pee is a turn off for me. I have had two occasions where folks had issues controlling their bladders mid orgasm, but because I was clear and not particularly affronted in the moment, the second time didn’t even end that session, just caused a short recess. It might not be sexy to discuss turn offs, but letting your partner know your dislikes how to address them with you should help. Things happen and be sure to make your partner knows that you will be emotionally safe to be around when they do.
Share relevant insecurities of your own, but only if they don’t belittle theirs. What I mean by that is if you have body insecurities that you struggle with that they know and are respectful of, express that you would like to do the same for them. Just avoid vague or minor insecurities that might make them feel you aren’t actually understanding their insecurity.
So my experience is from the perspective of someone who likes going down and who has had partners with pussies that were insecure about it. Patience is really key. The person with the insecurity needs to feel comfortable to start branching out and experimenting so focus on creating emotional safety for them. Let them know you love the thought, but will respect the boundaries they have set. Once that trust is there you can discuss options with them to lesson their anxiety.