Mouth to penis and penis to mouth in heterosexual relationships? 

I have seen where saliva is toxic to stds 

I'll first address fellatio, i.e. oral-penile contact. The main STD risks for the penile partner are nongonococcal urethritis (NGU), gonorrhea, herpes due to HSV1, and syphilis. NGU probably is the most common, but typically harmless; the NGU from oral sex is believed to result from normal oral bacteria that are harmless and most cases probably would clear up on their own if not treated. Gonorrhea is a risk primarily for men having sex with other men. Syphilis is rare in this situation and also most common with male partners. Genital herpes due to HSV1 is very rare after any single exposure, but oral herpes is quite common so it's something to keep in mind. For the oral partner in fellatio, the main risks are for pharyngeal (throat) gonorrhea, plus rare cases of syphilis and oral herpes due to HSV2. Note I have said nothing about either HIV or chlamydia:  both are theoretically possible but exceedingly rare from oral sex.

Cunnilingus -- oral vaginal contact -- is very low risk for both partners for all STDs. Gonorrhea, herpes, and syphilis certainly could occur, but in my 50+ years in the STD business, I've never seen a case of any of these acquired by cunnilingus. Here too, there are no significant risks for chlamydia or HIV.

In theory, HPV could be transmitted by all these kinds of contact, but this too is very rare for any single exposure.

In general, no STD testing is recommended after any single such exposure unless a) there are symptoms (like penile discharge or sores) or b) an oral sex partner is known for sure to have one of these STDs.

I hope this reply is helpful. Let me know if anything isn't clear.

HHH, MD

I am surprised by chlamydia- one place I read said it was easily transmitted by oral sex. 

You say it is rare- have you ever treated someone with chlamydia via oral sex?

A corollary is to avoid non-professional websites, like Reddit for example. Health discussions by and for people with a particular problem or worried about it tend to be highly misleading, often without regard to the available science. This certainly has been the case for oral chlamydia. Stick with professionally run websites or at least professionally moderated, like this one.

Have you ever treated someone with chlamydia from an oral exposure only? 

You said oral exposure clears rapidly- would you be confident that most if not all would be cleared by a month? 

Cleared by a month? It would seem you didn't read my blog-like reply, or missed that part. (Yes, usually.)

That completes the two follow-up comments and replies included with each thread and so ends this thread. I hope the discussion has been helpful. Best wishes and stay safe.