Examination of
the original Greek words for "natural" (physin) and "unnatural" (para physin) in Romans 1:26-27 reveals that the passage likely doesn't assert that men and women were somehow violating the laws of nature, but rather that they were going against their own nature, or
what is customary or expected. Being "para physin" (unnatural) isn't necessarily a bad thing in and of itself, as God went against the nature of the olive tree in Romans 11:24 when He grafted the wild olive branches into the good olive tree.
Romans 11:24 uses the same Greek words "para" for "natural" and "para physin" for "unnatural" that were used in Romans 1.
I read this passage as being about an idolatrous group of people (Romans 1:21-24), that God punished by "[giving] them over to their shameful lusts". (Romans 1:25-27, NIV).
Sexually immorality was expressly defined as giving into one's lust in
1 Thessalonians 4:3-6 which states, "For this is the will of God, your sanctification: that you abstain from sexual immorality; 4that each one of you know how to control his own body in holiness and honor, 5
not in the passion of lust like the Gentiles who do not know God; that no one transgress and wrong his brother in this matter, because the Lord is an avenger in all these things, as we told you beforehand and solemnly warned you." The nature of these lusts is not specified as being same sex lusts, but rather simply "the passion of lust" in general (See 1 Thess. 4:5).
Why would one assign special relevance to the fact this group's particular lusts happened to be between people of the same sex? Seems to me, what was bad about what the people were doing was that they were shamefully acting "in the passion of lust like the Gentiles who do not know God," rather than controlling their bodies in "holiness and honor". See again, 1 Thess. 4:3-6. If this same group had not controlled their bodies and engaged lustful sex in opposite sex combinations, it still would have been acting on their shameful lust in violation of 1 Thess. 4:3-6, wouldn't it?
What do you mean "no homosexual species exist in nature"? How about
the two male chinstrap penguins at the New York Central Park Zoo which display classic pair bonding behavior and have sex with each other? Or the wild male flamingos which have been observed to mate, build nests and raise foster chicks? Or the African bonobo ape -- a species which is virtually 100% bisexual and which egngages in sex which, 75% of the time, is nonreproductive. (See the same National Geographic article linked to in connection with the penguins, above, for info regarding the flamingos and bonobos:
http://news.nationalgeographic.com/news/2004/07/0722_040722_gayanimal.html).
On what do you base this statement? On what do you base your conclusion that sex may not be had for any other purpose than to reproduce?
I still don't understand what you base this statement on -- just because a certain proportion of the population may be homosexual doesn't mean that those people are "unnatural". Again, why does sex have to be about procreation all the time and for everyone? Where is this written in "natural law"? What is "natural law" anyway?