How do dating sites, cheating sites, and porn sites send e-mail requests to my e-mail account?

There are mnay reasons for spam, and you do not have to go to the site in order to get them. You can not completely stop all spam, but you can help to put a big dent in it.
First, try not to open any of them, that only makes them send more and more to you.
Check mark them and click on ‘Spam,’ this should stop those addresses from being delivered to your inbox.
Use your spam guard and block address.
Look for the ‘Options’ link at upper right corner of mail screen and click on it
Options /mail options
Click on ‘spam’ in the list on the left.
Make sure spam guard is on, by checking in the little box.
Then in ‘Blocked Email Addresses,’ enter the E-mail address you wish blocked, in the box after ‘Add a blocked address’ and click on the ‘add’ button, then click on save changes at top left of screen.
You will never get mail from that address again.
This should help you to control your spam.
Other tips:
Do not ever open E-mail from people you do not know; and unless you are absolutely sure who it is from then treat them as spam.
Opening spam alerts the sender that your address is a valid one and they send you loads and loads more of it to you.
Never, ever give out your account information or other personal information to anyone.
If you are on a site and you absolutely have to sign up for something, check the entire page to make sure you uncheck any lines wanting to send you any other info, notices, catalogs, ads, and the like.
Added detail:
Yes, spammers change one item in their addresses a lot, so you might try blocking the entire domain, by putting in
*@ thedomainyouwanttoblock.com
for instance: *@seeyou.com
Or, if they change the domain, instead of the company name you can block the name by putting in Imatheft_inc@*
Hope this helps