I don't know if this is the right place to ask but I heard that adult websites (porn) had viruses. I used to look up porn on my iPhone (I won't anymore) and I understand that there aren't viruses for iPhones yet, but if my iPhone gets a virus, can I affect other PCs on my network?
It doesn't really work that way. Any web site can provide active content, and any active content could be "malware". Virus, if you will.
Porn sites are no exception. Of course, reputable ones are less likely to stoop to malware to milk their customers.
One risk involved with an iPhone (or any smartphone) is that the site might try to trick you into subscribing to some service that will cost you money. This is sometimes done with the... well, I'll have to say complicity of the phone operator, which must supply information about your phone number. Then, any click anywhere on the page could potentially be crafted into a "request for premium service subscription", which would go straight on your phone bill.
Once a device is infected, what happens depends on the malware. Some of them actively attempt to replicate throughout the local network, and some of the most sophisticated will actually try establishing a contact to a "virus server" to download suitable code (e.g. if they detect a Mac OS machine on the network, they might try to download a Mac OS exploit, or try leaving an enticingly-named malware suitable for running on Mac OSes somewhere where the Mac's owner might stumble upon: file shares, DropBox folders, public areas...).
A first line of defense is an updated antivirus software. There are free versions too (AVG and Avira, I believe). Browsing with a secure browser (e.g. Firefox or Chrome with NoScript and adblocker plugins) is another good practice. Situational awareness - never click on something you're not sure of, and think before confirming any actions - is a definite plus.
If you're really paranoid there are "virtual secure browsing appliances" that run on VMware or VirtualBox, and allow browsing almost anywhere with a good certainty that whatever virus or infection will be unable to come out of the "sandbox" it's confined to. There are also strategies that essentially recreate the VM each time from a clean copy, and destroy it when you're finished, so that any virus you downloaded would be exceptionally short-lived. In this case, of course, you must be wary of what files you copy from the VM to the outer host machine, and scan them thoroughly with an antivirus.
Actually, if you're really paranoid you should buy DVD porn and watch it on a TV which is incapable of network connections :-) -- keep work documents and porn separated by a physical air gap.
The scenario you're proposing is possible (but probably not easy to execute).
Assuming you connect your iPhone to your home computer to transfer music files or pictures or backup your device, that would be an opportunity for software on your computer to try to open a file from your phone (possibly in the browser cache or downloads) and that file could maliciously exploit a vulnerability on your computer.
If you use something like DropBox that can sync files between your phone and laptop, a file downloaded to your phone could be replicated to your computer where you could try to open it.
Both of the scenarios I describe are a long shot. If you have a virus on your computer it probably came from a website or email. Lots of sites can push malware, not just porn sites.
"...even if there aren't viruses for iPhones yet..." -- This is a bad assumption.
Just to add some additional information to the current answers:
There is a high likelihood of exposure to malware when visiting porn sites, so you must take precautions when doing so. See http://www.techhive.com/article/2034864/just-how-much-malware-is-on-free-porn-sites-.html for a study on the prevalence of malware porn sites.
Even if the owner of a site does not place malware on the site, advertising material is typically served up from a different location, and may include malware. Even if you don't mind ads, it is worth installing an ad-blocker in your browser for security reasons.
The big danger in getting your mobile phone infected with malware is that said malware can start to use your phone services which, depending on your phone plan, can cost you real and significant amounts of money. This may all happen in the background without you ever having had to subscribe to any service. This used to be the case with PCs as well, with malware hijacking the modem and calling out to premium phone services, but this went away once broadband became the norm.
If you plug your phone into your computer (to sync to iTunes) anything is possible.
Incidentally, it's possible to send a signal from your computer to your phone with a special virus. This is very hard to accomplish, but since phones are often tethered, not inconceivable that this could be seen in the wild.