I have several points that I wish to make from the point of view of an unabashed marriage saver and one who indeed does believe that an exclusive marriage is the most healthy way to live.
First, there are more ways to know something than simply science. To use Dr. Klein's example, Hooker did indeed find no differences in MMPI profiles between heterosexual and homosexual males. However, anyone familiar with the MMPI should be willing to admit that it is a fallible instrument, and that scale 5 (masculinity-feminity) likely only measures range of interests. All that could reasonably be ascertained from her data is that straight and gay men did not differ in level of psychopathology as measured by the MMPI, or that the MMPI does not do a very good job of measuring homosexuality as pathology. The MMPI is an instrument that is designed to indicate pathology, not the determiner of it. We ultimately have to have some conception of normality and abnormality, not a measure. By way of example, Theodore Millon wrote that what constitutes pathology is anything that does not work to advance the species from within an evolutionary paradigm. Though he has not addressed this issue to my knowledge, it does not seem that homosexuality advances the species. I have yet to hear a reasonable argument on how it may do this.
I have discussed this for one reason - there are more ways of knowing than simply science. Common sense and reason seem to be viable ways of knowing and understanding the world. Others have suggested that art, literature, ect are also viable ways. It appears to me that each of these is useful in some way.
As a clinican, I work with families, mainly single-parent families. The children living in these homes often suffer from a variety of problems, including gender dysphoria, depression, anxiety, and behavioral problems. I know that many "healthy" couples have children who suffer from these problems as well, but it seems to be less frequent. The logical conclusion that I make is that lack of a father-figure in the home has at least something to do with childhood psychopathology. To return to Millon, having unhealthy children is not going to serve the advancement of the species.
Thus, the need for healthy children is at least one mark in favor of promoting monogamous marriage.