Okay, I'll start with the easy part. Nipples has nothing to do with it. Only mammals have nipples, not all animals. As for why males have them, well, simply because there's no reason for them NOT to have them. That's the simplest explanation of how evolution works.
As for how sexual reproduction evolved from asexual reproduction, that's a much tougher question. The simple answer is there is no answer. The evolution of separate sexes may not even be the same question. There's one theory with which I'm familiar. There are likely others, but I haven't looked that far into this question. There's a researcher, J.J. Crews, who studies lizards. The particular species he studies is interesting because they can be either parthenogenetic (I probably spelled that wrong), which means a haploid, unfertilized egg can develop into a lizard, if there are only females and no males present, or, if there are males present, they can reproduce sexually with fertilization of the egg by the male. Crews and his lab study these lizards because they may provide clues about the evolution of sexual reproduction (though not really the evolution of two sexes). A further interesting thing about these lizards is that in order for the parthenogenetic development to occur, two females engage in the same mating ritual as the female would otherwise engage with a male. This suggests there is something more than just fertilization that is important about the behaviors involved in mating for reproduction of that species. The theory arising from this work is that sexual behavior may have actually evolved before sexual reproduction. You can look up his articles for more details (or ask your librarian for help if you're not sure how to do this). Even if you don't have a lot of scientific background, many of his papers on the behavioral experiments are a reasonably straightforward read. I don't know what his recent work involves, so you might have to dig back to things published 10 years ago to find the stuff I'm talking about (I haven't followed that field for a long time now).
Now, how two sexes evolved may be a completely different question. I haven't thought much about it before. One direction you might want to explore in seeking some answers to this would be work in species such as certain reptiles that can develop into either males or females depending on environmental conditions, such as temperature. It seems like an obvious question now that I think about it, but I've never paid much attention whether there is a genetic difference between the sexes in those species or if it's entirely at the protein expression level.
