For the STD assignment:
http://www.geslachtsziekte.nl/content/default.asp
https://www.soaaids.nl/nl http://www.sense.info/soas/
The textbook: Bricks biology havo/vwo part 2
For the "What is your opinion?" assignment:
https://mens-en- health.infonu.nl/seksualiteit/
The textbook: Bricks Biology havo/vwo part 2
These links are all in Dutch. Of course you can also use other English sources!
Pay attention to the reliability.
A source is probably reliable if you find the same information on multiple sites. This does not mean that if you come across literally the same text on multiple websites, this is reliable! Often the information you find on weblogs or forums is not considered reliable. It comes from one person, who probably hasn't researched it.
If you are unsure about the reliability, ask your teacher to take a look.
Make sure you always convert the text into your own words, otherwise it is plagiarism and you will be judged on this!