Other than the condom, are there any other methods of preventing contracting STIs?
If so, do any of these also provide an effective form of contraception?
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Sexual intercourse naturally introduces microtears (small tears in the the epithelial layer) through which diseases can transfer between partners. One might think that additional lubrication would help with this, but the jury is most definitely out on that.
I think it's pretty safe to say that if there was another clinically verified way of preventing STDs, the CDC would have put it on this list. (For example, they note that there are specific things you can be vaccinated against, namely hepatitis B and HPV.) It strains credulity to think that there is a mechanism available that the CDC just forgot to mention there.
Having said all of that, I'll just mention the fact that there are a variety of types of condoms. If you are in a situation where you need to use them (e.g. non abstinent and not in a long term monogamous relationship), you might be able to find a brand or type that works better for you.
EDIT: A comment has pointed out that a cervical barrier method can reduce the risk of transmission of certain types of STIs. Since that does not show up on the CDC list I mentioned, I feel that it's worth bringing up. By no means is that a "safe substitute" for a condom (and I don't think anyone is implying that it is). However, as a risk reducer, it should be added to the list.