While not explicity answering your question, When did controlling cultural sexuality become such a major issue for Christians? by /u/BBlasdel from the FAQ may help until a more specific answer can be provided

There are a few areas that should be clarified before answering this question.

First, you note a number of scriptural references where biblical figures are depicted having premarital sex. The assumption is that these depictions tacitly approve of the behavior of the individuals. However, that's not necessarily the case - while many modern media approaches do take that approach, imputing those motivations to Biblical authors may lead us into troublesome spots. For a modern example of an approach to media where acts considered immoral by the creators/audience/approvers must be met with in-story consequences or condemnation, see the Comics Code.

So let's look at the stories you mention:

• Abraham and Hagar: After Abram sleeps with Hagar, we get some rough consequences. Hagar begins to despise Sarai, Sarai mistreats Hagar, and Hagar runs away. Her son, Ishmael, is destined to "live in hostility" and "be a wild donkey of a man." Later, Sarah asks Abraham to send Hagar and Ishmael away. God tells Abraham to do so, and that he will make a nation out of that son as well - although Issac (Abraham's son with Sarah) will be the son that his offspring are reckoned through. When it appears that Hagar and Ishmael are about to die, God reveals a well to her, and they go off to live in the desert. The next we hear of Ishmael, he is burying his father, though he does not receive a blessing from him. His descendants, Genesis 25 relates, "lived in hostility toward all the tribes related to them." So while neither Abraham nor Sarah (nor Hagar) are explicitly condemned by God in the story, neither are they given a seal of approval. And the author paints a picture that doesn't exactly imply that what Sarah and Abraham did was good - or that it led to good things. Abraham's tryst with Hagar results in the near death of a mother and child, division in Abraham's house, and a nation perpetually at war.
• Jacob and Bilhah and Zipah: It should be the antecedent to Jacob sleeping with the slaves of his wives was the jealousy of Rachel. In the rest of the chapter, we read about Rachel and Leah basically pimping out Jacob to each other - swapping sex for mandrakes. Again, no explicit disapproval - but in general, those kind of judgements aren't present in the text. This section of Genesis doesn't contain much commentary. The bulk of it is one event after another, plain and simple narrative where the voice of God rarely intrudes. Many things explicitly described as sinful or immoral do not receive explicit condemnation, so it's not totally surprising that extra-marital sex also doesn't receive that treatment. However, the general tone of this story in particular doesn't exactly paint the act as noble. Particularly with the trading of sex for mandrakes, it's almost comical! Either way, it would be a stretch to say that the behavior of the parties involved is laudable or to be emulated.
• Samson and the Prostitute: Again, we don't get commentary on Samson's act with the prostitute. We're told he sleeps with her, we see he retains his strength after doing so. And immediately after we get his tryst with Delilah, where he reveals his secret to her and ends up losing his strength - the Lord had left him. This results in his capture and blinding. After his hair regrows, during a pagan sacrifice, Samson is called out to entertain - he prays to God for strength and revenge, and collapses the temple. The act with the prostitute is a single sentence that opens the finale of Sampson's story, where we see his foolishness on display, leading to his abandonment by God, capture by the Philistines, and death. The author's intent seems less to be "sleeping with prostitutes is fine," and more "look at this idiot." It should also be mentioned that this happens in the Book of Judges, who's refrain is "In those days, there was no King in Israel, and everyone did what was right in their own eyes." In many ways, this book represents the nation's moral nadir. It isn't a series of behaviors to be emulated - in fact, it ends with an oath-breaking mass murder and rape, before the author reminds us why everything went wrong - anarchy and individualism run amok.

A question that might best answer your question is "what does the Bible mean by "sexual immorality?" There's not one blanket answer, because some parts of the Bible are more specific than others. For instance, Leviticus contains clear prohibitions against adultery, incest, homosexuality, bestiality, etc. Other texts, like Genesis 2, seem to imply an understanding of sex and marriage as inherently connected (a man and his wife are joined by becoming "one flesh"). But in the New Testament, we start getting much broader. Many passages (such as 1 Corinthians 818, 7:2, John 8:41, etc.) refer broadly to "sexual immorality" - typically, the Greek word "porneia." So what would the readers of the New Testament have understood porneia to mean? Would it have just included the specific and explicit commandments in the Torah? Probably not. The word porneia was translated into Latin in the 300s as "fornicati" and other related words, from whence we get fornication - any (typically consensual) sex outside of marriage. We know that the Roman audience of the New Testament authors would have understood porneia/fornication to refer to acts including prostitution, but also just plain sex outside of marriage. We can see that reflected in the story of Mary and Joseph, where Mary's pregnancy is met with shame, as she and Joseph aren't married yet.

Tl;dr:

1. Biblical narratives aren't necessary meant to be emulated, and we can't assume tacit approval by the author.
2. The audience of the New Testament's gospels and letters would have understood sexual immorality to include all kinds of fornication, or sex outside of marriage.
3. The New Testament particularly speaks broadly in condemnation of sexual immorality, though taboos against pre-marital sex existed prior to it's writing, and can be inferred from the Old Testament as well.