Hebrews 11 does in fact lend some credence to the "perpetual virginity" theory.
Heb 11:32 And what more shall I say? For the time would fail me to tell of Gideon and Barak and Samson and Jephthah, also of David and Samuel and the prophets:
33 who through faith subdued kingdoms, worked righteousness, obtained promises, stopped the mouths of lions,
34 quenched the violence of fire, escaped the edge of the sword, out of weakness were made strong, became valiant in battle, turned to flight the armies of the aliens.
***
39 And all these, having obtained a good testimony through faith, did not receive the promise,
40 God having provided something better for us, that they should not be made perfect apart from us.
Because it suggests that both Samson and Jephthah "obtained a good testimony through faith". In Samson's case that makes sense because he repented at the very end of his life and was presumably restored but there is no mention of Jephthah repenting after completing his vow.