The best sources are always the ancients' writings themselves. If you read an historian, you will always get their interpretation of those sources. That said, Goldsworthy, Hunt and Mommsen are good historians, though the first two focus more on Roman military history.
Try Cicero, who wrote on mores of his time, like his On the Laws or On the question of Good and Evil. For another perspective, try the Atheist Lucretius' On the nature of Things.
For the Greeks, Plato wrote Phaedrus and the Symposium, that both address the idea of Eros, in which pederasty and homosexuality are mentioned.
The Greeks were far more forgiving of such things, and a sexual relationship between an older man and an adolescent boy were considered fairly normal. You even see this with Alexander the Great, who had a male lover in Hephaestion and also the Eunuch Bagaos, in addition to his wives and his great love Roxana. Usually though, this was in addition to a normal wife as well.
The Romans were against homosexuality. They contemptuously called Greeks "boy-lovers" and thought it shameful. Even here though, there were gradations. The active 'male' participant was thought less shameful than the passive 'female' one.
This is why the Hellenophilic Emperor Hadrian scandalised traditional Roman morals when he took Antinous as his lover. This is also what lies behind the slur on Julius Caesar that he had acted as catamite to Nicomedes IV of Bithynia, probably spread by his enemies, and that his soldiery took up with gusto as part of the traditional mockery of the commander during a triumph.
Romans considered Adoption as valid as if someone was born into the family. This is why adoptive children sometimes took precedence over blood children, such as Tiberius after Augustus, or that adoptive children were expected to have traits of their adoptive families as well, such as with Scipio Aemialianus Numantinus.
On Paul, I wrote a thread about a year ago speculating on where he got his Roman citizenship, that addresses things like adoption:
The Roman Citizenship of Paul
