Can anybody explain to me the meaning of this sentence?
A "stretch mark" is simply the wrinkles a woman has, on her belly, after being pregnant and having a baby. (You know, the flabby skin that can be there, after having a baby.)
A "fetish" is an unusual sexual desire. So, a "foot fetish" is being sexually attracted to feet. An "ear fetish" is finding ears really sexy.
Now, it is normal, for comic effect, to talk of a ridiculous fetish. For example, "I have a road sign fetish" (for some bizarre reason, road signs turn me on) or "She has a potato fetish" (potatoes arouse her sexually).
If you understand the phrase "Leave open the possibility..." you now understand the sarcasm or comedy that is being made here.
(It's likely a woman writer, sarcastically saying something like: "Well, I sure will not be attractive after I have the baby! Of course, there's the possibility of a stretch-mark fetishist!")
More context would have been nice. It turns out this is from Youth in Revolt and the earlier statements in the conversation are:
Stretch marks occur on a woman's stomach when she's pregnant. The skin has to stretch to accommodate the baby. After birth, even though the skin goes back to its previous size (more or less), there are still marks. Nick is stating that maybe she'll find a man who find stretch marks sexually exciting. A fetishist is someone who focuses on a particular item (like women's shoes) or body part.