hopefully, having bug sex without the bugs able to lay eggs just feels like they shouldn't have dicks in the first place to me. There's a pregnancy mechanic? What is it exactly cause the page seems to be missing that tag?
You need to increase fertility with a potion you can craft in your inventory, the potion is made using some flowers you find on the beach (I also found them deeper in the forest and in the fields but the beach is the closest location where you can find two of them)
After that, have vaginal sex with the creatures. I think the chances of vaginal sex are increased if the girl, the creature or both are aroused. You can become aroused by either masturbating (default M, and wait for the animation to finish) or crafting and drinking the arousal tincture (red potion you see in that screenshot above, to craft it you need to double click the icon and to drink the potion you need to drag and drop it from your inventory into one of those slots you see at the bottom, select it and hold down mouse button to drink it).
The creatures can become aroused by throwing the arousal potion at them, it also increases the creature's fertility (the purple one you see on the right in the screenshot above). You can find the flowers to create the potion on the left side at as you first enter the forest
I am kinda confused about this whole system because sometimes it's vaginal sex and sometimes it's other positions even after drinking the potions. It's pretty clear that positions change depending on which potions are used but I'm still not sure which one does what.
I found it more consistent to do vaginal sex with the cockroaches. I used a fertility potion on my character then used the purple splash potion on a cockroach and it initiated vaginal sex in the pronebone position. The cockroaches themselves can be found deeper in the forest and higher up in the mountainous areas but they're pretty fast and go all over the place so you might not find them in the same areas I did.
If after a while you get a message saying "I feel strange" or something like that it means she's pregnant. Her belly will grow over time until she eventually gives birth, but there's no animation or anything. It just fades to black. Here's what it looks like when she's pregnant