HoldForm does not Hold Form for fractions sometimes

Use HoldForm applied to each fraction to keep the fractions from combining.

HoldForm[1/2] HoldForm[3/4]

to produce $$ \frac{1}{2} \frac{3}{4} $$

or

HoldForm[(1/2) (3/4)]

to produce $$ \frac{3}{2 \times 4} $$

Using TeXForm produces the desired LaTex code.

(HoldForm[1/2] HoldForm[3/4]) // TeXForm
(* \frac{1}{2} \frac{3}{4} *)

Addendum

Simpler is

Infix[f[1/2, 3/4], "\[Times]"] // TeXForm
(* \frac{1}{2}\times \frac{3}{4} *)

which also provides the times sign. $$\frac{1}{2}\times \frac{3}{4}$$

Second Addendum

z1 z2 /. Times -> Cross /. {z1 -> 1/2, z2 -> 3/4} // TeXForm

also produces the desired output. (This is based on the third Answer to 39061.)

The behavior you observe is due to the formatting rules associated with Times. Please start by reading my answer here: Returning an unevaluated expression with values substituted in. We can apply a similar technique here though the result is not quite as desired if we merely block Times during Box creation. We get:

$\left(1*\frac{1}{2}\right)*\left(3*\frac{1}{4}\right)$

This form is due to the internal format of 1/2 and 3/4:

Hold[1/2, 3/4] // FullForm

Hold[Times[1, Power[2, -1]], Times[3, Power[4, -1]]]

One way to handle this is to post-process the Box form yield the format we desire:

SetAttributes[hf, HoldAll]

MakeBoxes[hf[args__], fmt_] := 
 Block[{Times}, MakeBoxes[HoldForm[args], fmt]] /. 
  RowBox[{"(", RowBox[{n_, "*", FractionBox["1", d_]}], ")"}] :> FractionBox[n, d]

Now using hf in place of HoldForm:

hf[1/2*3/4] // TeXForm

\frac{1}{2}*\frac{3}{4}

Formatted:

$\frac{1}{2}*\frac{3}{4}$

I am posting a second answer because I am now taking a very different interpretation of your problem. In a comment below my first answer you state:

Your function seems to correct one type problem with fraction. But I am more looking for something able to display TeX in the exact form I write them. Probably MM is not the right tool to use. I am disapointed.

I assumed that you were looking for TeX conversion of arbitrary expressions generated by (evaluation in) Mathematica but if instead you simply want TeX for expressions in "the exact form I write them" you may be able to use Strings, e.g.:

The string was created using standard input methods. \[Times] was entered with Esc*Esc.

Here is the input in copyable form:

"\!\(\*FractionBox[\(1\), \(2\)]\)\[Times]\!\(\*FractionBox[\(3\), \(4\)]\)" // TeXForm

And the output formatted by MathJax:

$\frac{1}{2}\times \frac{3}{4}$

Critically this method avoids interpretation of your raw input into e.g. Times and Power, thereby bypassing those "pretty printing" rules that were changing your expression in an unwanted way.