Is it possible to get a relevant link and relevant link text from a weight loss site to a nude teens porn site and have google consider it an on topic link? Short answer is yes. Long answer is yes too but it takes some work to make it a relevant link.
So how would I do it? Fairly simple inf you think about it.
Putting lots of text that’s relevant to your page can make even a non relevant page relevant. You see links can’t be done away with and yet you can have relevant links from sites that are total opposites or badly out of niche. If the link isn’t relevant the SE gives bad resutls.
A weight loss page could link to a nude teens page and be relevant. If the SE dismiss it they won’t give the best resutls. If they include it and it’s not relevant they won’t give the best results. Damn, what’s a poor SE to do. Whoa, wait, how can a veggie page be relevant to nude teens.
So, ever wonder why some places demand 200-500 chars when you submit a link? Because the words you are using should be relevant to what you are linking to.
BTW, those 200-500 chars should have several relevant keyword phrases in them. I just hope the link above mine and after mine is also relevant text. IMO if directories were doing it right your link would be somehwere in the area of 75%-95% of of the way thru your description. Then it would always be relevant.
Why? Because SE now read the words before and after the text link to make sure the link is relevant. I’d also bet they read more in front of the link than they do after it altho I’ve never tested that and have no proof. It’s logical but I’ve been logical and wrong before.
The words surrounding the link now mean more than the niche the link came from does. Nice if you can have both but either will work.
I might write the text for the nude teens as somethin like, “Now here’s a teen girl who looks as though she might need a little bit of weight loss to look better. Obviously this hot little teen looks good but she could lose a few pounds. If you want to check out a pic of this teen nude and see how she looks and why she needs to lose a few pounds you can see her hot naked body for yourself. Just see how this nude teen looks with her clothes off. She could stand to lose a couple of pounds.”
Now I have a relevant link from a weight loss site to a porn site. Bummer huh? 
NEVER link from a non adult site of your own to an adult site. At least not as a recip. NEVER. Even if you can make it relevant. SE view adult sites as bad “hoods”. Nice people don’t go to the “hood”. We know better than that but SE don’t.
Google won’t penalize you for any incomong links you have as they know you can’t control that. However they may penalize you for linking to bad neighborhoods. For sure you’ll probably get penalized if it’s a recip link. However a link in a comment is a different kettle of fish. It probably won’t get penalized as long as it’s relevant.
I believe the direction google is heading, to decide what links are relevant, is reading the surrounding text, and, if it’s relevant and the link is in the text, it will be considered a relevant link.
You could do the same thing if I made a post about BLONDE NUDE TEENS that’s the topic of the post and any other posts should relate to that according to current thinking. However if you use the above technique you could link to redheads, fat or anything else and make it a relevant link with the text in the comment that surrounds the link.
Obviously we would still be linking porn to porn but it’s a different niche and a relevant link. That’s what’s important. It’s also what blogs are very handy for.
