When we first see Ophelia in act 1.3, her brother, Laretes, is warning her to beware of Hamlet. After doing so, her father, Polonius does the same. Therefore from the start we know Ophelia and Hamlet have been having a relationship, so much so, she has to be forbidden to commnicate with Hamlet.
If Ophelia has had sexual relations with Hamlet, do you really think she is going to risk telling her family and ruin her reputation?

When told she has been spending a lot of time alone with this guy. Ophelia professes: "he [Hamlet] hath importun'd me with love / In honorable fashion." Later however, Hamlet requests that Ophelia, "Nymph, in thy orisons be all my sins remembered." Orisons are prayers. If Hamlet has importuned Ophelia in honorable fashion, why should his sins be remembered in her prayers? The implication is that they have sinned together.

	In act 2.1, Ophelia is now afraid she has caused Hamlet some madness and reveals to her father, "My lord, as I was sewing in my closet, Lord Hamlet, with his doublet all unbraced; No hat upon his head; his stockings foul'd, Ungarter'd, and down-gyved to his ankle;" doublet all unbraced = jacket unfastened stocking foul'd, ungarter'd, and down-gyved to his ankle = fallen down to his ankles.

In 3.1 Ophelia is set up to test Hamlet. She tries to give back "rememberances" but Hamlet is on to this trick. As Hamlet declares he never loved her, Ophelia is heartbroken, "I [Ophelia] was the more devieved." Hamlet instructs Ophelia to "Get thee to a nunnery" which can mean convent (for her baby) or brothel, which still implies that she has lost her virginity. "Why wouldst thou be a breeder of sinners?" "Or, if thou wilt needs marry, marry a fool; for wise men know well enough what monsters you make of them. To a nunnery, go, and quickly too. Farewell." "And quickly too"...what is the hurry?

As the play is about to be started Hamlet asks Ophelia if he shall "lie in your lap?" to which Ophelia refuses and Hamlet restates the question to, "I mean my head upon your lap? "Do you think I meant country matters?" "That's a fair thought to lie between maid's legs."

And finally we get to the songs.
	"To-morrow is Saint Valentine's day, All in the morning betime, And I a maid at your window, To be your Valentine. Then up he rose, and donn'd his clothes, And dupp'd the chamber-door; Let in the maid, that out a maid Never departed more."

"By Gis and by Saint Charity, Alack, and fie for shame! Young men will do't, if they come to't; By cock, they are to blame. Quoth she, "Before you tumbled me, You promised me to wed. So would I have done, by yonder sun, An thou hadst not come to my bed."