This book must have been published before 2001. I read it as a High School freshman (EvergreenPark HS, IL). I only have a vague memory of it. It involved a man raised (and probably created) by an alien race. He discovers the photo he thought was his mom is a just a model from a magazine page. They take him to earth and he may have fallen in love/infatuation with a reporter or military woman. He dives into an underwater cave with the earth woman and she tells him the truth behind his alien family. Very vague, I know. Thanks!
This is Frederick Pohl's Homegoing (1989). Sandy, the human, is brought back to Earth.
What I remember happening in the underground cave (a building in flooded Manhattan?) is that Sandy has intercourse with the lady, who gets violently ill. It turns out it is an allergic reaction, and it stems from the fact that Sandy's body has an haphazard biochemistry due to the fact that he's not at all a foundling, but a gengineered clone of a dead astronaut recovered by the aliens (the lizard-like Hakh'li), and shares some DNA with them.
The picture of his "mom" is indeed not his mom's at all; it is actually that of Marilyn Monroe. From the book:
“But you did have that picture of her. Well, Ham put it on television to see if anyone would recognize it. A lot of people did. But that picture isn’t of an astronaut, Sandy. It’s of a movie actress from the last century; her name was Marilyn Monroe.”
One curious detail about the Hakh'li that you might remember is their graphical poetry - poems written so that the ideograms double as a picture. Lysander (Sandy) writes a poem about Earth that ends describing the Moon with a circle and the words, more or less,
Your Moon's nice
This is the page from Goodreads.
A similar story, but not the answer (yet it fits the same subject, so it might be useful to include) is C. J. Cherryh's Cuckoo's Egg (1985). There, the young Thorn is cloned from a dead astronaut's body, and raised with the alien hatani worldview. This is done by the aliens so that they may communicate with Earth and clear the incomprehension that resulted in the astronaut's death in the first place. Here, too, there is a picture of the boy's mother which is anything but, and the problem of his adolescential sexual urges.