Only a matter of definition
It is only a matter of definition. When it comes to sex, there is such a diversity of mating system that it is very hard to make a good definition that won't be challenged by limit cases.
Sexual reproduction can typically be defined based on the presence of meiosis (but again this will lead to limit-cases), on the need for two distinct parents + meiosis, or on the need for two distinct parents which have different gender (anisogamy) + meiosis.
Diversity of reproductive systems
At the end of the day, what one wish to call sexual reproduction does not matter too much as long as the definition is being made clear especially when dealing with potentially limit-cases.
If I remember correctly the intro book to evolutionary biolgy called Evolution by Futuyma makes a good overview of the diversity of reproductive system. Having a vague understanding of this diversity will help you undermine your hope of finding a clearly defined definition for sexual reproduction.
Applying the definition of wikipedia to parthenogenesis
Here is the definition from wikipedia
Sexual reproduction is a form of reproduction where two morphologically distinct types of specialized reproductive cells called gametes fuse together, involving a female's large ovum (or egg) and a male's smaller sperm
Using this definition and applying it to the specific case of parthenogenesis, one would conclude that parthenogenesis is not a form of sexual reproduction (despite the presence of meiosis). Note by the way the even within the term parthenogenesis, there is quite a bit of diversity (see the wikipedia entry for parthenogenesis).
what is the evolutionary explanation for this phenomenon(parthenogenesis)?
I will ignore this question because
- A post should always be limited to a single question
- This question is very broad
- There is not a single answer to this question given the diversity of types of parthenogenesis.
For a start on the subject, you might want to have a look at Jaenike and Selander, 1979 for example.