Given that this is an article about sexual morality by a Christian author, you can have a pretty good idea of what the blanks will say. Christians who hold to traditional values believe that sex is only morally acceptable in God's eyes if it occurs within a marriage. Christians such as Lane do not believe that it is acceptable for a couple to engage in sexual activity, even if they are deeply in love and committed to one another, until they are formally married.
This is what Lane is saying in this article. Therefore, two of the words for the blanks above are "exclusive" and "marriage." The other word, put there to emphasize that he does not believe in same-sex marriage, is "heterosexual." The full passage is:
Sex is rightfully enjoyed within the context of an exclusive, heterosexual marriage.